Hi Carrie! Sounds like you are planning a fun trip! First, please check your dining reservation to confirm whether you have reserved a meal at the Crystal Palace or at Cinderella's Royal Table. Both restaurants are in the Magic Kingdom: Cinderella's Royal Table is inside Cinderella's Castle and is a character meal featuring the princesses. Crystal Palace is a character buffet that features Winnie the Pooh and friends. Both restaurants are fun character dining experiences, but if your daughter wants to dine with Cinderella and her princess friends, make sure you have reserved a meal at Cinderella's Royal Table, NOT Crystal Palace. (Although a restaurant with Palace in the name does sound like a place for princesses, you won't find them there, and I don't want your daughter to be disappointed.)
At Cinderella's Royal Table, many of the young girls dress up in their favorite princess gowns and tiaras for their meal in the castle. Not every girl does, but many do, and I think your daughter is the perfect age to wear her princess dress to the castle. If she enjoys dressing up, definitely pack her princess gown and crown!
Hope this helps, and I hope you have a magical vacation!