are you doing the castle package? or just doing the hair and make up? Costume may be too hot, but if she gets her hair put up like a princess, then it will be cooler. Make up might melt. What time of day is the reservation?
Our dd, who was 4 at the time, did BBB in April and it was hot. She did the castle package, and after the photo session she was ready to take the costume off. Her hair was up in a bun, and the make up was done lightly and we didnt have any problems. The hair managed to stay three days before we took it down and washed it. The humidity will be higher in July.
But I guess the bottom line is, is this something your dd wants to really experience and is it worth it to you to let her, even if he doesn't last long?
For us, we are glad we did, the photos from the photo session are priceless. We only got so many pictures in the package, but all the photos that were taken were put on our photopass!