The Library of Congress has an online collection of Public Domain Children Books available

This special collection presents children’s books selected from the General and Rare Book Collections at the Library of Congress. The collection includes classic works that are still read by children today, and lesser-known treasures drawn from the Library’s extensive collection of historically significant children’s books. The books in this collection were published in the United States and England before 1924, are no longer under copyright, and free to read, share, and reuse however you’d like.

Source: About this Collection  |  Children’s Book Selections  |  Digital Collections  |  Library of Congress

These books are in the public domain, which means students are free to remix and share their own creations made with the pictures, text, or characters of these books.