During the 18th century and for much of the 19th, there wasn't a whole lot of American literature for children. And when children's books did get published, they weren't designed for pleasure. Books were for schooling or for teaching religious and moral lessons—with properly serious illustrations chaperoning the text.
This somber mode continued through the Civil War. And then it went poof, dispelled by artists who became children's illustrators by happenstance. By the end of the 19th century, the art in kids' books had become madcap and zany and irreverent. From the postwar period, one can trace the imagery and style that are familiar from the classics of one's own childhood.
Click herefor a slide show on the history of children's book illustration in the United States, based on Timothy G. Young's new book, Drawn To Enchant.