We took our son to a local (ish) amusement park this summer to get an idea of how he'd be. We've been watching Disney movies/ reading Disney books to get him acquainted with the characters. There's also a Disney sing a long from the 80s called
Disneyland Fun that is great because it basically walks you through a day at Disney.
Worse comes to worse, you hold off on the character meet and greets until your child is comfortable with them. Don't force it, let them lead
And if there is a melt down, hey, maybe you can record it and get on AFV!