My daughter is significantly older than yours, but I'll answer.
She has had it done at age 9 and 10. She absolutely loves it and I enjoy watching her enjoy it from the time she gets dressed to the last shot of the photo session and into the park with the compliments.
She's tenderheaded, but I usually mention it at the beginning and she's done well with the bun style. She doesn't want to try a different one. Both times she kept the hair for 2 days before I washed it out.
3rd and probably final appointment is in April for her birthday. She will age out and is sad to do so. After that we'll likely switch to Pirate's League since her brother is now old enough to try that.
I really don't have anything bad to say. I don't have experience with the costumes as we brought clothes from home and didn't even wear Disney princess clothes. First time we had a long formal gown I got on ebay as we had CRT after. 2nd time she had a sparkly tank top and pettiskirt and denim vest. Next time will be some type of party dress or gown as we'll have CRT again.