Hi Michelle,
The
Voyage of the Little Mermaid is a theatrical show where guests sit in chairs in a theatre seating. If your daughter is familiar with the movie, she'll be more prepared for the scarier parts of the show and will also know that it ends well, which may help to keep her calmer if she does get scared.
I will admit that it does have some moments which can be scary when the villain is on stage (oh that jealous Ursula!). What may help you to know if it is okay for your daughter is to watch it in advance on Youtube, which has a collection of videos guests have shared. Overall, though, I do really think it is a delightful attraction.
If you do visit and have to leave, just let a cast member know who will be happy to safely assist you out. In fact, you may want to try to sit near the end of a row or toward the far back as a precaution to make it easier to leave if you have to.
Have a wonderful trip! Please let us know if we can help again.