We had my daughter's hair done last year when she was 4. She HATED it. I am sure the entire island of
Castaway Cay could hear her screaming. It was a very unpleasant experience. So be aware that if you have daughters that hate having their hair brushed or pulled at they will NOT like having it braided. They have to pull really hard and it hurts to have it done. I think it was around $40. We didn't have any appointment or anything like that. We just walked up to the place where they braid hair.
But the end result once all the tears were dry was that my daughter LOVED her braids. It was also wonderful to not have to do a single thing with her hair our entire vacation. We went to Disney World for a week after the cruise and her braids lasted the entire time. Her hair was out of her face and she looked really cute. I was glad we did it, but I won't put her through that again!
Here's a picture from several days after she had it done. (Taken in Cinderella's Castle Suite!)