Hi Kristin,
My little girls love the Princesses too! You'll definitely want to take her to
Bibbidi Bobbidi Boutique. She can be transformed into her favorite Disney Princess, complete with hair, nails and make-up. This is such an enchanting experience. If the expense is a concern, she can always bring her favorite Princess dress with her to wear to the Boutique.
I also recommend that you make an Advanced Dining Reservation for one of the Princess meals. My favorite is Cinderella's Royal Table inside Cinderella Castle. This is very popular though, so if you can't book it you should try to get the Princess Storybook Dining at Akershus in Norway (Epcot). She'll be able to visit with 4-5 Princesses in each location. Call 407-WDW-DINE to book this as soon as possible.
You can also pick up a Time Guide when you enter each park. Specific character meet and greet times and locations are listed on there. In the Magic Kingdom you can see
Rapunzel and
Princess Tiana. You can also often see Snow White, Ariel and Belle there. If there is a certain character that you are looking for, stop by Guest Relations and they will be able to tell you where to meet them (if they are greeting guests that day).
Also, if you are going to Hollywood Studios, be sure to watch
The Beauty and the Beast - Live on Stage Show as well as The Voyage of the Little Mermaid.
Please let me know if I can help you further and have a wonderful vacation,
Kaylene