Alright, I've lifted enough stuff from Cabinet Magazine's interview with artist Leonard S. Marcus, who gives a fascinating, conversation-sized history of children's books, from the 17th century to Harry Potter. Marcus is an expert on the subject and yet another Cabinet interviewee who says, "that's a good question":

The 1960s were a turning point for children's literature. For one thing, it was then that most editors and librarians finally realized that most children's books were about the life of the white middle class. An article in the Saturday Review of Literature in 1965 entitled "The All-White World of Children's Books" caused a lot of people to think about what they had been doing. Until then, the children's book world had been so self-enclosed, with middle-class book publishers selling their books to middle-class librarians. A few years earlier, in 1962, a picture book called The Snowy Day, by Ezra Jack Keats, had been published for very young children. It was set in Brooklyn and showed a little black boy walking out in the snow and having a great time. It had nothing to do with being black, but the fact that he had dark skin made it unique for its time. Other books followed, and suddenly picture books seemed, to a very limited extent, to become more integrated than before.