I think let the birthday child choose what the day consist of and then build some special items into that. Such as a pre-ordered cake. A few years ago I found a shirt for my daughter that said Birthday Girl in sparkly letters. We were able to go to Disneyland that summer for her birthday and she go so much attention from the cast members. If you are creative you could make a shirt.
This LINK will take you to the complete birthday at WDW information here on the DIS!
There are lots of things birthday related at WDW as you can see there. Some may even give you ideas for things to do on your own with your children too...
Carol, Thank You very much for providing the link. We will be looking into all suggestions as they come in and I'm sure that it is going to be a very exciting and joyful day.