The research collection of the Cotsen Children's Library is a major historical collection of rare illustrated children's books, manuscripts, original artwork, prints, and educational toys from the 15th century to the present day in over thirty languages. The collection has important holdings of materials in the English, Chinese, Czech, Dutch, French, German, Greek, Hebrew, Italian, Japanese, Latin, and Russian languages.

Among the collection's treasures are an early-Coptic Christian schoolbook; medieval manuscripts; incunables; two scrapbooks assembled by Hans Christian Andersen; drawings by Edward Lear, K. F. E. Freyhold, and Samuil Marshak; many of Beatrix Potter's famous picture letters; early editions of the fairy tales of Madame d'Aulnoy, Charles Perrault, and the brothers Grimm; one of the largest collections of children's books published by John Newbery and his successors; Soviet Constructivist children's books; American dime novels; moveable books; jigsaw puzzles; and educational playing cards.