Hi Martha,
While Cinderella's Castle is cool, you can celebrate anywhere at the Walt Disney World Resorts with your princess.
Two places that comes to mind are
Crystal Palace and
1900 Park Fare at the Grand Floridian. The Crystal Palace has Winnie the Pooh and friends who have a party around the restaurant. This restaurant is a buffet which has wonderful food and lots of fun. We celebrated my daughters' birthday here all the time when they were little.
1900 Park Fare has Cinderella and her friends. Set in the Grand Floridian, this is now my favorite character meal as the night we had dinner, Cinderella's stepmother and sisters entertained us all. You can even have your picture taken with them! This buffet is smaller and more select but still many options for the younger princesses!
If you would like a special cake, you can call Cake Hotline at 407-827-2253 at least 48 hours in advance. Otherwise, you will recieve a dessert with a candle on top at both restaurants to sing and blow out candles with your daughter.
Remember to tell your hostess you are celebrating a very magical birthday!
Beth