The Cinderella ideas all sound wonderful, however, this cake is for a three-year-old! small plastic birds and other tiny plastic things should not be used, as they are a choking threat!

In our local Big Y grocery superstore with an in-house bakery(others may offer this service, too) you can get any design painted on an edible paper with edible 'inks' you could easily have a picture of Cinderella copied right onto the top of a nice cake.
Simple is better, especially for a three-year-old--just how much to you remember of your third birthday?

Good luck, be careful, and have fun!