Hello, Robin! It's so fun to watch children explore the Disney Parks in costume. In fact, you'll probably see a lot of little pirates and princesses on your Walt Disney World vacation!
Not only is your daughter permitted to wear her beautiful princess dress to the Disney Parks, she is encouraged! When my daughter dresses as Princess Ariel or Rapunzel, Cast Members and Guests alike greet her with such welcoming phrases like, "Hello, Princess!" and "Welcome, Princess!" It really makes her feel so special and provides that extra touch of magic that makes our Disney vacations so memorable.
The costume restriction that you mentioned in your question only applies to adult Guests of the Walt Disney World Resort, and does not apply to your young daughter. If you would like to read more about appropriate attire for visiting the Disney Parks for Guests of all ages, make sure to visit the FAQ page regarding "
Dressing Recommendations."
I hope that you and your daughter have a very magical vacation at Walt Disney World, Robin! If you have any additional questions, please do not hesitate to contact me here at the Moms Panel. Happy planning!