I took my twins when they were three, mostly because my DD7 was going. One twin did great, loved the whole thing, kept the hair style in for 2 days although we didn't go swimming those days (too much fun stuff in the parks, we can swim at home!)
The other twin was a disaster. We had the first appointment time, and the fairy godmothers do a little ceremony to open the shop. Despite having her sisters with her, she started crying and clinging to me. She calmed down and wanted her hair done, but only on my lap. So she sat on my lap for the whole thing. Then she cried when they combed the gel in, so they only put a little in. Then after it was done she started pulling on it. She had it all undone and out by that afternoon. So for her it was a waste.
You know your little one better than anyone. Will she sit nicely for a stranger combing her hair? How sensitive is her scalp? How does she do getting her haircut? I could have predicted my daughter wouldn't leave her hair in because she would always pull at ponytails and mess them up, but I was OK with that going into the experience. I was surprised by how upset she was though because she is normally very friendly and she was very much looking forward to this. Fast forward three years later and she's the only one of my three who says she would like to do this again!