Hi Jill,
I'm sure your princess loving granddaughter will love being at WDW for her birthday. My daughter is the same age and loves princesses as well. Last year we treated her to the castle package at
Bibbidi Bobbidi Boutique. It is a little girls dream come true. The girls are treated just like princesses from the second they walk in. They receive a princess dress, accessories, and a makeover. The look on my daughter's face while one fairy godmother did her hair and another one painted her fingernails was priceless.
After the makeover session, the princesses get a professional photo session. The girls just glow after the entire experience. Afterwards as you walk around the park don't be surprised if WDW castmembers and even other guests compliment your daughter and refer to her as a princess.
You can even top off your day at one of the princess themed character meals. Try Cinderella's Royal Table, Akershus, or even 1900 Park Fare.
Have a magical trip - Michelle