Hi Amy!
What a magical time you're going to have with your 2 little Princesses!
While I'd usually recommend ALL the parks, at your daughters' ages I think I'd recommend using 3 full days to visit each of: Magic Kingdom, Hollywood Studios and Animal Kingdom.
My very favourite "combo" for a Birthday Princess would be to visit the Bibbidi Bobbidi Boutique & then enjoy a Princess meal while she's all dolled up!
I'd recommend booking a Princess Package at the Magic Kingdom's
Bibbidi Bobbidi Boutique location inside Cinderella castle. You'll find the pricing information and package details at the link provided above. Make a reservation right away (it's a popular place!) by calling (407) WDW-STYLE. Start with the 'Coach' package - as you can opt to add on some extras upon check-in at the salon. Your Princess can choose to wear her own dress or Princess attire or can purchase a head-to-toe Princess ensemble or accessories at the boutique. Bring your camera as there will be plenty of magical moments to capture!
A perfect complement to this experience, if you'd like, is to dine with Cinderella that day. You have 2 options available to do so without straying far from Magic Kingdom. Choose from breakfast, lunch or dinner at
Cinderella's Royal Table inside Cinderella Castle... or ... MY favourite - pop out the front gates & take the resort boat launch or resort monorail to the Grand Floridian to dine at
Cinderella's Happily Ever After Dinner at 1900 Park Fare.
Feel free to reply if you have any more questions I can help you with Amy!
I hope that your Princess' birthday is her most Magical day ever!
Kirsten