The Society of Children’s Book Writers and Illustrators is a non-profit ,
501 (c)3 organization which is  one of the largest existing organizations for
writers and illustrators. It is the only professional organization specifically for
those individuals writing and illustrating for children and young adults in the
fields of children’s literature, magazines, film, television, and multimedia.

A meeting of great minds. Part 2 here.

The personal site of Harold Underdown, freelance editor and author of
"The Complete Idiot's Guide to Publishing Children's Books." This site has
articles full of advice for aspiring children's writers and illustrators,
recommendations on publishing houses and agents, and dos and don'ts for
submitting manuscripts and getting your story noticed.

This simply titled and simply designed website from author Patrice
Sherman is a great place for beginners to start. The author gives a
rundown of the different kinds of picture books, how to give and receive a
critique, and how to properly format a manuscript.