Hi Noelle! Cinderella's Royal Table is one place a girl should never feel self-conscious about dressing like a princess! If you can't wear your tiara in the Castle, where can you wear it?
If your daughter still enjoys dressing up, then by all means, she should dress up in her princess gown for breakfast with Cinderella. Although more younger than older princesses dress up, there will be girls of all ages and sizes in gowns and crowns, and no one will make her feel out of place. If, however, she no longer enjoys dressing up, she certainly won't be the only girl dining in her theme park clothes. My daughter is almost 8 and she enjoyed dressing in her princess costumes at Cinderella's Royal Table when she was younger, but on our most recent trips, she chose not to, and was perfectly comfortable in the Castle in her theme park clothes. (I'm a little sad my daughter no longer wants to dress as a princess, but I'm happy that she still likes to dine in the Castle with them!) Just let your daughter choose to dress the way she feels most comfortable, and she'll have a great time either way!
Have a magical time with the princesses!