My top recommendations for aspiring children's writers:

• Join your local SCBWI chapter.
• Check out Verla Kay's Children's Writers' and Illustrators' Chat Board (aka the Blue Board)

For sheer coolness and innovation:

Resources especially for young writers:

• Stone Soup, a magazine by young writers and artists
• Speak Up Press, an online journal that publishes young writers' work
• The Mystery Writers of America's annual Joan Lowery Nixon Award mystery-writing contest just for young writers. (The deadline is usually in January or February.)
• Writing contests and an ezine at Amazing Kids
• Scholastic's resources and contests for young writers and their teachers

Information I've developed for newsletter articles and conference handouts:
(You'll need Adobe Acrobat Reader to open them.)

• Tips for receiving critiques
  
• Blogging basics for children's writers
• Blogging basics for all writers
• Perks and perils of self-publishing
• Writing from the subconscious
  
• The art of the ending
  

Develop thick skin like this fellow. Keep writing. Keep biting.