Writer's Digest published a list of the 101 Best Websites for Writers. Though they were not primarily focused on writing for children, there were several that I thought were helpful:
Winning Writers includes some poetry contests,
Cheryl Klein gives her perspective as a senior editor at Scholastic's Arthur A. Levine books,
Cynsations is Cynthia Leitich Smith's blog
WOW! Women on Writing includes market info and contests
Wordsmith is just plain fun for word lovers.