My son has an October birthday and really enjoyed having a Halloween themed birthday party. When he moved into his tween years I remember that he really, really wanted to show a scary movie but we felt that he was not ready for most of the horror movies that were available. What resulted was a search for a movie that was scary but not too scary for pre-teens.
We found a few great older movies, like Creature From The Black Lagoon and Walt Disney's Watcher In The Woods and, more recently, we discovered Coraline is a great choice, as long as you check out the age recommendations first. Even Coraline can be disturbing for some tweens. If in doubt, I say read as much as you can about the age recommendations for a movie and consider previewing it for your children.
A Halloween movie birthday party for tweens IS a great idea. Just be sure you know the age appropriateness of the movie you pick. Most horror movies are simply not suitable for children who are pre-teens.
Quick Links:
Not Too Scary Movies For Tweens or Pre-Teens on Halloween
Before Green Gables