Following are depictions and portraits of boys and girls from the Middle Ages and Renaissance. Because the goal of this survey is to provide useful information on a realistic view of children's clothing, I have avoided most allegorical or overly anachronistically-dressed children. I've also included links to extant clothing worn by pre-17th century children.

Take a look at the BNF's online resources for L'enfance au Moyen Âge for late medieval (mostly 15th century) examples, or Children's Portraits for more 16th-19th century examples. More discussion on this topic at the Garb The Child LiveJournal Community. For 17th century examples, see Costume Focus 1660-1715: Children Research Gallery.

• Child's tunic with ducks, Coptic, 6th or 7th century
• Tunic of Don Garcia, first half of the 12th century
• Clothing of Doña María, 1235 (more photos here, here, here, and here)
• The Manesse Codex (UBH Cod. Pal. germ. 848), 1300-1330
  Because of the way the Manesse Codex was drawn, it is sometimes hard to tell whether the shorter figures are intended to be children, or adults who have been shrunk to fit the space available. Illustrations featuring children include those of Von Wissenlo (fol. 299r), Herr Reinmar der Fiedler (fol. 312r), Meister Friedrich von Sonnenburg (fol. 407r), and Der Litschauer (fol. 422r).
• Jacob blessing Ephraim (fol. 8v), and giving matzot and haroset to children (fol. 15), The Golden Haggaddah (British Library Add. 27210), early 14th century
• A boy plays a rebec at the Marriage at Cana, The Queen Mary Psalter (British Library Royal 2 B. VII, fol. 168v), c. 1310-1320
• Several illustrations in the BNF Fr 152 Bible historiale (14th century)
• Several of the illustrations in the marginalia of Romance of Alexander (Bodley 264), c. 1338-44, feature children, including the puppet show on folio 54v.
• Boy punished by his teacher (fol. 214) and a teacher and pupils (fol. 541), Omne Bonum (British Library MS Royal 6 E. VI), c. 1360-1375
• Job with ten sons and daughters, Bible historial (MMW 10 B 23, fol 246v), 1372
• Herjolfsnes 44, a dress worn by a girl in Greenland, approximately ten years old
• Herjolfsnes 86, a cap found near a boy's grave
• Herjolfsnes 62, a child's garment
• The salt-seller (fol. 5r), mulberries (fol. 10r), sweet cherries (fol. 11v), pears, butter, dill (fol. 31v), the east wind (fol. 57r), Tacuinum Sanitatis (ÖNB Codex Vindobonensis, series nova 2644), c. 1370-1400
• Rain, Tacuinum Sanitatis (BNF Nouvelle acquisition latine 1673, fol. 95v), c. 1390-1400
• De mulieribus claris (BNF Fr. 598), beginning of the 15th century
  The combat of Eteocles and Polyneices (fol. 36)
  Hecuba weeps for her children (fol. 48v)
  Execution of Olympias (fol. 93v)
• The assassination of Barsine and Hercules (fol. 114) and Orodes II chooses his successor (fol. 116v) in De casibus (BNF Fr. 226), first quarter of the 15th century
• Sweet cherries (fol. 8v), beets (fol. 24v), dill (fol. 29), horehound (fol. 33v), violets (fol. 36), millet (fol. 46v), raisins (fol. 54), the east wind (fol. 55), butter (fol. 59), and chicken eggs (fol. 63v), Tacuinum Sanitatis (BNF Latin 9333), 15th century
• Hector's family begs him not to go into battle, L'Epître d'Othéa (British Library Harley 4431, fol. 135), c. 1410-1411
• Funeral sculpture of Béatrice de Beausart and her son Guillaume de Melun, 1419
• Virgin and Child by Rogier van der Weyden, c. 1440
  While not all paintings of this theme show realistic & workable infant clothing, this one is especially practical.
• Family and servants, Liber de proprietatibus rerum (BNF Fr. 135, fol. 193), c. 1445-1450
• The left panel of The Seven Sacraments Altarpiece by Rogier van der Weyden, 1445-50
• Presentation of Christ in the Temple by Stefan Lochner, 1447
• Aristotle teaches Alexander, History of Alexander (BNF Fr. 9342, fol. 7v), mid-15th century
• The family of Job, book of hours with proverbs (BNF NAL 3134, fol. 104), mid-15th century
• St. Anne and the Three Marys in the Book of Hours of tienne Chevalier (BNF NAL 1416), 1452-1460
• A peasant boy with his family, Armorial de Revel (BNF Fr. 22997, fol. 344), 1456
• Portrait of a Young Girl by Petrus Christus, after 1460
• A sitting man gives bread to three boys (lower right-hand corner), Factorum et dictorum memorabilium (KB 71 D 42, Book 4: Chapter 1, fol. 202v), 1462
• The Presentation in the Temple by Hans Memling, 1463
  The two little girls are easier to see in this detail.
• A family, Aristotle's Politics, Ethics, and Economics (BNF Fr. 22500, fol. 6), 15th century
• Speculum Historiale, 1463: crossing the Red Sea (BNF Fr. 50, fol. 55v); the story of David (BNF Fr. 50, fol. 74v); the story of St. Eustace (BNF Fr. 50, fol. 380); Abraham sends Hagar away (BNF Fr. 50, fol. 44); St. Mark and St. Marcellian (BNF Fr. 51, fol. 46); the story of St. Paphnuce of Heraclea (BNF Fr. 51, fol. 158v); the miracle of St. Martin (BNF Fr. 51, fol. 297); the story of St. Germain (BNF Fr. 51, fol. 431)
• Illustration for Psalm 80, showing David with musicians and dancing children, in a psalter (KB 131 F 18, fol. 86v), ca. 1465-1470
• Francesco and Sigismondo Gonzaga in The Meeting in the frescoes at the Ducal Palace in Mantua by Andrea Mantegna, c. 1471-1474
• The Donne Triptych by Hans Memling, 1475
  The donor's daughter is easier to see in this detail from the center panel.
• Amphiaraus of Argos meeting Eriphyle (fol. 26r), the discovery of Opheltes' death (fol. 29r), and Andromache begs Hector not to return to battle (fol. 111v), Roman de Thès (PML G.23), 1474
• Marcus Furius Camillus receiving the schoolmaster of Falerii and his pupils; the schoolmaster is stripped and the children drive him back to their city with sticks, Factorum et dictorum memorabilium (KB 66 B 13, Book 6: Chapter 1, fol. 289r), c. 1475
• A boy plays with a pull-toy boat on wheels, a book of hours (PML M.1001, fol. 48r), c. 1475
• The City of God (MMW 10 A 11), c. 1475-1480
  St. Ambrose speaking to two boys and a girl (fol. 13r)
  Two boys among a group listening to St. Paul (fol. 13v)
• The Knight Cifar (BNF Esp 36), fourth quarter of the 15th century: fols. 14, 14v, 16, 16v, 17, 32, and 32v
• Portrait of a Boy by Pinturicchio, 1481-3
• Triptych of the Family Moreel by Hans Memling, 1484
  See also the details of the side panels: Willem Moreel and his sons, and Barbara van Vlaenderberch and her daughters.
• Self-Portrait at 13 by Albrecht Dürer, 1484
• Boys putting up a tent for Leopold the Mild, from the Family Tree of Bamberg by Hans Part, c. 1489-1492
• An Old Man and His Grandson by Domenico Ghirlandaio, 1490
• Portrait of Urschelin Burgkmair by Hans Burgkmair, c. 1500
• The Wealthy Man by Jean Bourdichon, from Les Quatre États de la Société, c. 1500-1510
• Hans Holbein's sons Hans and Ambrosius in a panel by Hans Holbein the Elder, 1504
• Triptych of Jan Des Trompes by Gerard David, 1505
  The side panels depict the family of the donor; this detail provides a clearer view of his first wife and their daughters. The rear of the wings includes a portrait of the donor's second wife and their daughter.
• Portrait of a Young Girl by Albrecht Dürer, 1507
• St. Anne Altarpiece by Quentin Massys, 1507-08
  Several boys depicted in contemporary dress. Note especially the children of Mary Cleophas; baby Joseph seems to be wearing a sort of bib, embroidered with a cross.
• Triptych with the Holy Kinship and the Trinity with members of the Merode-Petershem family, southern Netherlands, c. 1510
• Portrait of a Girl by Albrecht Dürer, 1515
• Signboard for the Schoolmaster Myconius by Ambrosius Holbein, 1516
• Boy with Blonde Hair and Boy with Brown Hair by Ambrosius Holbein
• A Prince of Saxony by Lucas Cranach the Elder, 1517
• A Princess of Saxony by Lucas Cranach the Elder, 1517
• Conrad Rehlinger and his Children by Bernhard Strigel, 1517
• Triptych of the Adoration of the Magi by Jacob Cornelisz van Oostnanen, 1517
• A Young Girl of Cologne by Albrecht Dürer, 1520
• Madeleine of France by Jean Clouet, c. 1520
• The Dauphin François, Son of François I by Jean Clouet, ca. 1523
• A Family Group by Bernardino Licinio, 1524
• Child's caftan, second quarter of the 16th century
• Two pairs of farmers with children at a covered table by Pieter Brueghel the Elder, c. 1525-1569
• The children of Christian II of Denmark by Jan Gossaert, 1526
• The Darmstadt Madonna by Hans Holbein the Younger, 1526 and after 1528
• Rural scene by Marten van Cleve, 1527
• The Artist's Family by Hans Holbein the Younger, 1528
• Portrait of a Man with Three Sons by Barthel Bruyn, 1530
• Family Portrait by Maerten van Heemskerck, c. 1530
• Fol. 1v of the Hennessey Book of Hours, c. 1530-1540
• Young Girl with Astronomic Instrument by Jan Gossaert
• Portrait of Arrigo Licinio and his Family by Benardino Licinio, 1535 (detail)
• Peasants Merrymaking at a Village Kermesse by Hans van Wechlen, c. 1537
• Edward, Prince of Wales by Hans Holbein the Younger, 1539
• Portrait of a Young Girl, Lucas Cranach the Elder, 1540
• Henry Brandon by Hans Holbein the Younger, 1541
• Charles Brandon by Hans Holbein the Younger, 1541
• Edward, Prince of Wales, with Monkey by Hans Holbein the Younger, 1541-1542
• Bia by Agnolo Bronzino, before 1542
• Clarissa Strozzi by Titian, 1542
• Edward, Prince of Wales by Hans Holbein the Younger, 1543
• Count Alborghetti of Bergamo and his son by Giovanni Battista Moroni, c. 1545-1550
• Red-headed Youth Holding a Drawing by Giovanni Francesco Caroto, before 1546
• Giovanni della Volta with his Wife and Children by Lorenzo Lotto, 1547
• Giovanni de'Medici by Agnolo Bronzino, 1549
• The Shelley sons, 1549
• Boy's embroidered shirt, c. 1550
• Infant's knitted sock, vest and mitten (mid-16th century?). Baby clothes knitted in soft white wool; the mitten has a contrasting band, probably of naturally black sheep's wool. Another photo here.
• Knitted and felted cap with a decoratively slashed brim; 16th century
• Drawings of the children in the French royal family: Hercule-François, unidentified child, François II, and Charles IX
• Boy's leather jerkin, c. 1550-1600
• Infant's coif, c. 1550-1600 (V&A 7523-1861)
• Eleonora of Toledo with her son Giovanni de' Medici by Agnolo Bronzino, 1550
• Don Garcia de' Medici by Agnolo Bronzino, 1550
• Francesco I de' Medici by Agnolo Bronzino, 1551
• Maria de' Medici by Agnolo Bronzino, 1551
• Livia da Porto Thiene and her daughter Porzia and Iseppo da Porto and his son Adriano by Paolo Veronese, c. 1551
• The dauphin (later François II) by Lonard Limosin, 1553
• Marguerite de Valois by François Clouet, c. 1555
• François II (by François Clouet?)
• Much of the work of Pieter Bruegel the Elder features children, including Children's Games (1559), Peasant Wedding (1568), Peasant Dance (1568).
• The Pelham sons and daughters, c. 1559
• Giuseppe Gualdo and sons by Giovanni Antonio Fasolo, c. 1560
• Peasants by the Hearth by Pieter Aertsen, 1560s
• Boys at various ages in the early 16th century, from the Schwartz Trachtenbuch, 1561. (Note that the BNF illustrations are in color, but are from a 1704 copy of the manuscript.) There is some contrast between the childhood clothing of Matthaus (who grew up in the early 16th century) and his son Conrad (who grew up in the 1540s). Thanks to Sophia Kress for her help with these manuscripts!
  Matthaus as a sickly three-year-old (see also BNF Allemand 211, fol. 9)
  Matthaus learns the alphabet at the age of five years and four months (see also BNF Allemand 211, fol. 4)
  Nine-year-old Matthaus has run away, and makes a living begging for bread and working for cowherds (see also BNF Allemand 211, p. 13)
  Matthaus plays in the street at the age of eleven years and 8 months (see also BNF Allemand 211, fol. 7v)
  At the age of 12, Matthaus goes through a phase of wanting to be a monk, and holds Mass in his bedroom with his own miniature altar (see also BNF Allemand 211, fol. 9)
  Conrad, aged 1 to 3 years (one with a walker, and one wearing a pudding-cap and apparently being held up with leading strings?)
  Conrad, at age eight and a half, playing various games in the street
  Conrad, age ten, plays in the snow
• Portrait of Juana of Austria with a young girl, 1561
• Lady Katherine Grey and her son Lord Edward Beauchamp, c. 1561-1563
• Pierre de Moucheron, Merchant of Middelburg and Antwerp, and his family, 1563
• Henry Stuart and his brother Charles Stuart by Hans Eworth, 1563
• Fête at Bermondsey by Joris Hoefnagel, 1569
• Charles IX by François Clouet, 1570
• Detail from a portrait of a woman and two children by Giovanni Battista Moroni, 1570
• Portrait of a Family with Arms of Mosto and Costanzo (Detail) by Tintoretto, 1570s
• Infantas Isabel Clara Eugenia and Catalina Micaela by Alonso Sanchez Coello, c. 1571
• Swaddling Band, Italy, c. 1575-1600; a later swaddling band can be found in the V&A's Access to Images (search for Museum Number B.879-1993; a christening outfit can also be found at Museum Number 900-1864). Also, a 17th century tailclout at the National Museum of Childhood.
• Foemina Mediocris conditionis in Silesia (Commoner from Silesia) from Habitus praecipuorum populorum by Hans Weigel, 1577
• Antonius Anselmus, His Wife and Their Children by Marten de Vos, 1577
• Arabella Stuart, 1577
• Sketch by Adam Elsheimer, c. 1578-1610
• A young boy by Jacob Willemsz Delff I, 1581
• Otto Venius painting, surrounded by his family, 1584
• Oliver Pelham, 1584
• Two Young Men by Crispin van den Broeck, before 1590
• Miniatures of unknown children by Isaac Oliver, 1590: V&A P.145-1910 and P.146-1910, and an infant
• Prince Karel of Arenberg and his family by Frans Pourbus de Jonge, c. 1593
• Unknown lady with two children, 1595
• Boy servants in the Album Amicorum of a German Soldier, 1595, including: Donzela Senese, Sposa Senese, Marchese Napolitana, and Donzela Senese
• Barbara Gamage, Lady Sidney, later Countess of Leicester, with two sons and four daughters, c. 1596
• Philip of Savoy, Prince of Piedmont, 16th century
• Thomas Stewkley, 1601
• Double portrait of a brother and sister by Cornelis Ketel