Hi Greer! A tiara and a princess gown are definitely in order for lunch at Cinderella's Castle! My daughter used to love dressing like her favorite princess before dining at Cinderella's Royal Table, and she got so much attention as she walked through the Magic Kingdom dressed in her royal best.
For a really special treat, you could make an appointment for your niece at the
Bibbidi Bobbidi Boutique before your lunch with the princesses. At the Boutique, Fairy Godmothers-in-Training style little princesses' hair, paint their nails, and apply shimmering makeup and a tiara or other hair accessory. Princess gowns are available for purchase at the Boutique, or you can bring a gown that you bought previously at one of the shops at Walt Disney World, such as the World of Disney, at the
Disney Store, or at other retailers, such as your local discount stores or online sites like Ebay.
Wishing you a magical lunch with Cinderella! Come visit us again at the Moms Panel if you have more questions! We'd love to help!