From Ptak Science Books:
I think that I can safely say that artwork by children does not make very many appearances in Western art prior to the 20th century. Nor do the originals--considering the ephemeral nature of the effort at art by children, their work just don't seem to survive. Some of that reason--particularly in America--was the scarcity of materials for kids to produce art with: paper was not inexpensive, and neither brushes and paints. Crayons, invented for chubby and reachy fingers, were not invented for the mass market until 1903. (Crayola sold eight crayons in a box for a nickel. The colors? Black, brown, blue, red, purple, orange, yellow, and green.)
Then of course the artwork would have to be saved, somehow, for generations. As much as it would be fascinating to find artwork done at age 4 by your great-great grandparents, it would have to survive the cleansing tendencies of four generations of clean-up. End result: there's just not that much antiquarian chldrens' art floating around.
It also doesn't appear as art in artwork. It is possible to find numerous examples of kids' scratches in stone and such in ancient graffiti, but it doesn't appear as elements of fine artwork, or, for that matter, in book illustration.
There is an example however in Thomas Truman's The Nurse's Rhyme Book, a New Collection of Nursery Rhymes, published in Philadelphia in 1847. The book is filled with unusual illustrations and fantastic ornamental borders, all used in support of some odd, scary, mean and occasionally pretty mid-century posies meant as night-time entertainment for the young ones. Our prize is found on the very last page, the final slug of an illustration to a more-finely illustrated book: a coy, small boy, holding an example of his art, seemingly drawn on a framed slate. He looks happy, pleased, proud to me--on the one hand he is interested in sharing his achievement and on the other is really too shy to share, an emotion I've seen from time to time with my girls.
As a matter of semi-fact, this is a rare emotion to see displayed in art, the too-shy-to-share routine saved more for fluttery self-conscious Victorian grown-ups more so than for children..
link: Ptak Science Books: Missing Images in Art: Depictions of Art by Children