On the invite I would use some type of wording that might be found in the movie. Like: "Have you seen Tinker Bell and The Great Fairy Rescue? Help us celebrate ______'s Birthday and rescue Tinker Bell" and/or "Pixie Hollow is recruiting you to be the newest fairy. Your mommy or daddy is welcome to join if they'd like or they may drop you off in Pixie Hollow and return at ____."
I don't think it would be rude to tell them it might be more fun to watch Tinkerbell beforehand but it might be unnecessary. I think if you put a lot of thought into the invite and make it all about Tinkerbell and the Fairies they might get the hint.
I hope she has a wonderful birthday! I'd love to hear what ideas you have come up with, I am thinking about a Tinkerbell party for my daughter as well =)