If you want to write for children or teens you should definitely join the Society for Children's Book Writers and Illustrators (SCBWI)
Here are some other useful sites:
Cynthia Leitich Smith - Cynthia's site is packed with fantastic information, author and editor interviews, you name it!
Verla Kay - Verla Kay's website is another useful mine of information
FOR TEENS WHO WRITE:
PUSH - an imprint of Scholastic started by author and editor David Levithan, Push has an annual novel writing contest for teens in grades 7-12.
Teen Ink - an e-zine written for teens by teens
FOR PRE-TEENS WHO WRITE
Stone Soup - publishes stories and art from kids up to the age of 13.