Australian author Dee White has started a new website called Writing Classes for Kids (& adults). You’ll find info about writing, writing tips, links to useful websites, writing competitions, and writing classes for kids and grownups (available as $5 downloads).
Check out the info about the first competition on the Writing Classes for Kids website. There’s a category for 8 to 12s and there are book prizes—the competition is free and entries close 30 November 2011. Good luck!