IMHO I would not get her a costume, per se, to wear to CRT. We went to CRT with DS6 in March over Spring Break. You have a photopass photographer take a picture of your family and Cinderella, or just your daughter and Cinderella, you decide. When we went, in the restaurant, Belle was there in her everyday dress, the blue one with a big white apron, and Jasmine was there in her Arabian type outfit, Snow White in her primary colored dress, and the fairy god mother. So think about what you would like your daughter to look like next to these characters in the pictures that you are sure to take.
When we went, we saw several little princesses all dressed up in their costumes. Fancy Belles, Cinderellas, etc etc., but two of the cutest little girl sisters we saw were all dressed up in very princess like dresses that were obviously not character dresses. They had the big, long flowing dresses with big bows on the back, beautifully dressed up hair, and maybe even a tiara, with hair ribbon flowing down. They were adorable. They looked like princesses that were attending another prinesses ball, if you see what I mean. They were unique and there wasn't another little girl that looked anything like them, but they fit right in with the mood of the event. So, that is what I would recommend. Dress your princess up in style. You may be able to find some great dresses on sale now that Easter has past.
If you wanted to add to the story of a princess eating at another princess' house, you could fashion an invitation for your daughter as if it were coming from "Cindy." If you are really crafty, or into scrapbooking, or handy on the computer, you could make a short little story book about the event and set the tone for it by the story that you tell. That may be over the top for some, but for some people, they can whip that kind of thing out in a heart beat...but again, just IMHO.
