Our daughter, Ashley, turned 8 and she requested a rainbow theme birthday party/slumber party so, of course, that's what she got. Our children are a little spoiled : )
I made her a rainbow doodle cake for her birthday which you can find directions for here http://www.whisk-kid.com/2009/08/say-it-with-cake.html and I just added fondant to it with this recipe http://thecookduke.com/fondant-recipe/ and let her color on it with food color markers which were bought at Wal-Mart. This was my first time making and eating fondant. It's not very tasty, but it can be peeled back if you don't like the taste. I didn't really like the buttercream icing either that the recipe calls for. If I were to make it again, I would just use box cake mix batter and color that and use canned frosting. I think the kids would have liked it better that way. Btw....yes, I did put my rainbow colors in the wrong order. I guess I got tired, lol.
All the kids had a blast!