The Official Jane Yolen website - Jane Yolen, one of the most prolific writers of children's picture books, gives helpful advice to would-be writers along with links to helpful websites.
Harold Underdown - His website is chocked full of information for all kinds of writers, though he specializes in children's fiction. The article, "Getting Out of the Slushpile" should be read by everyone!
What, exactly, qualifies a book to be YA? Or J? The Write4Kids website can tell you! In addition to many great articles on writing children's fiction, this site offers tools for writers, links, and other helpful information.