Since the party starts at 7:00, I'd be concerned about how late she'd be able to stay up to enjoy the festivities. My kids would have been nodding off early in the evening at that age, but if your dd can stay up later, then perhaps that's not an issue.
If she's pretty good about napping, maybe you can schedule a later (and hopefully long) nap that day before the party. Then, she'll be awake enough to enjoy the characters and the trick-or-treating (photo ops with the characters!) for a while. Make sure she's in a comfortable costume--nothing too warm in Florida and nothing itchy.
If you go, have a good time!