Just do what SHE wants. She's the princess during that dining moment, so she decides what to wear. I've just been there last week and what I've seen? About 1/3th of the girls in princess dresses, 1/3th in other caractre outfit (shirt, short, shoes, hairribbons, combination of these or singular) and 1/3th in their every day clothing. She can wear whatever she wants and not feel out of place. With a dress on she'll get compliments on the dress for sure! With a princess-shirt she'll get comments on that to. None of the above? She'll get enough of attention of the princesses and not feel weird at all. Because I must say, the princesses really took their time with each and every table, even going back to a table if they had just come after the princess would have hit their table or they were just getting some food. I've seen worse at caractre dining.
Definately bring out your camera. When I was there Belle -in the yellow dress- was the princess taking the pictures in the front, so that would be THE oppertunity for your DD.
Also at her age maybe it's an option to go "in between" if she wants to go for a dress. Not the costume ones that are sweaty and itchy, but a real nice dress she's got anyway that makes her look pretty, do up her hair in a way she thinks is special, maybe even give here a tiara and she'll feel Royal anyway big time. They don't need the specific Belle/Sleeping Beauty/whatever princess dress for that. This way you don't need to buy a dress that she might not want to wear (just bring something she's allready got) and she can pick right then and there; what do you want to wear to dinner, the shirt or the dress?