Hi Patricia,
I am sure your little princess is beyond excited to head to the magic. She is at a perfect age to experience so much that there is to offer. My daughter is about the same age and our last visit to Walt Disney World was perhaps one of her favorites because she could do so many more of the attractions and did not get as tired.
I highly recommend going to Magic Kingdom first among the four parks. It is my personal favorite, as it is the most iconic with Cinderella Castle, and the walk up Main Street, U.S.A. still gives me chills of excitement every single visit, even at my 40 years of age. Fantasyland is full of fairy tale fun including Princess Fairytale Hall, Enchanted Tales with Belle, and Under the Sea-Journey of the Little Mermaid. My all-time favorite ride is Peter Pan's Flight. Other favorites at Magic Kingdom are Splash Mountain, Pirates of the Caribbean and Space Mountain.
If you haven't already, I would strongly suggest getting dining reservations for either
Be Our Guest Restaurant or
Cinderella's Royal Table. These dining spots are the perfect place to be treated like royalty. Keep in mind that these hugely popular restaurants do tend to book up very quickly and can be booked up to 180 days prior to your visit either online or by calling (407) WDW-DINE (939-3463).
I hope this helps you determine the best itinerary for your sweet daughter. You will have an unforgettable trip, no matter what the order.
Happy Planning!
~Allison L