I was 12 when I first saw the midnight showing (at CityWalk!), and it was SO much fun. It was already my favorite movie before then and it was so exciting to experience the showing. I've seen it quite a few times since there and I love it more and more. I will say that I was a little shocked at a lot of the language (even though some of it I didn't quite get yet). So it's a good idea to prepare her that the movie is much tamer than the audience!
You might want to get to the theater early, as the tickets tend to sell out.
I'm so glad there are parents out there as open-minded as mine were!

Have fun!
Fun aside note: You guys mentioned the old Alien ride at Disney.. did you know Tim Curry provided a voice for that? I though that was a funny coincidence