My granddaughter turned six at WDW and we planned a big day around her. Started with Bibbity Bobbity Boo for a Birthday Princess makeover. She was gorgeous and had a sash that said "Birthday Princess" and she was acknowledged by many, many people. Cast Members and kind guests that made a fuss over her. We had breakfast at Norway with the princesses who also made a huge fuss.
Lunch we had at the Plaza and I had preordered a special birthday cake with a photo of the castle on it and her name. The whole restaurant sang Happy Birthday to her.
We finished off the evening with a special viewing of the fireworks. What a day.
You can also get balloons delivered to the room in the deluxe resorts. Just make sure everyone knows it is her/his birthday and everyone gets in on the act.