Listed below are my favorite sites for children's writers. I hope you find them helpful.

SmartWriters - for writers and readers of children's literature
The Society of Children's Book Writers & Illustrators
The Children's Book Council
The Children's Writing Resource Center - Be sure to check out the message board
Harold Underdown's Purple Crayon
Author Verla Kay's site has loads of information for new writers
Author Cynthia Leitich Smith's site is an excellent resource for both authors and educators, with an emphasis on multicultural literature.
Author Sheri Gilbert's site - full of helpful tips
Right Writing - Writer and former ICL instructor Terry Whalin gives practical tips and provides helpful articles for writers of all genres.
Robin Friedman - Check out Robin's many editor interviews
America Writes for Kids
The Institute of Children's Literature has an Rx for Writers section, with helpful articles that change monthly.
The ICL's Transcripts section, where dozens of chats between the site editor and experts in the field are archived. You'll find information on any subject imaginable pertaining to writing for children.
Farrar, Straus and Giroux Books for Young Readers
Henry Holt Children's Books

If you're a fan of children's poetry, or want to learn more about writing it yourself, check out the following sites:

Meadowbrook Press - This site is chock-full of funny poems (including Read 'Em & Rate 'Em) and advice for kids, writers, and teachers.
Ted Scheu - (that poetry guy)
Jeff Mondak
Kenn Nesbitt
Charles Ghigna - aka "Father Goose"
Kristine O'Connell George has excellent advice on her site for budding poets
CJ Heck - Poetry and short stories by author CJ Heck. Site also has mazes, tongue twisters, quotes, silly poems to finish, essays.