Alex Knight has a great tutorial up on simple Parent/Child navigation in Silverlight.
The best part about Silverlight is it provides heaps of great tools to use, so people like me who are completely graphically minded and a little bit developer stupid are still able to create sweet menus and the like.
This blog will show a simple way to create a menu that has all the main categories visible and when clicked will show your child links for that category, and when clicked again will make them disappear.