Welcome aboard, Janet! One of my daughter's favorite experiences on the Disney Fantasy was being transformed into Snow White in the
Bibbidi Bobbidi Boutique. That is something she will remember forever! Your daughter will have a very magical time during her makeover there, I'm sure!
If you are able to change her makeover time, I would do that. The Main Dining seating begins at 5:45 p.m., and though guests do not need to line up to enter the restaurant at exactly 5:45, I wouldn't go too much later than that, as your service team will be waiting for you, and after the Main Dining, they do need to turn over the restaurant to prepare for the second seating.
If you find that you cannot get a different time for the makeover, just keep it, and use that night as a time to try out the buffet for dinner at
Cabanas, order a special room service meal (which is complimentary!), or grab a casual bite up by the pools. Though you certainly can find meals elsewhere, your daughter might really enjoy being the "Belle" of the ball at your regular seating that night, so if possible, I'd choose another time for the makeover. Also, you don't want to rush your daughter through that magical experience, so you will want to have plenty of time to enjoy it before having to be at dinner.
I hope that clears things up for you, Janet. Please let me know if I can help with anything else as you continue to plan your magical cruise!