The Last Exorcism is pretty solid for a newish film. Though probably a little over the head for younger viewers.
Alright here's Aaron's Intro to Horror: (off the top of my head)
The Ring and The Grudge if you want Japanese re-makes, if you can handle subtitles watch the originals. Alien for an all-out scarefest. Poltergeist if they want a haunted house movie. Evil Dead II for zainey, scary fun. Silver Bullet for some 1980's Corey-Gary love. The Devil's Backbone Bill De Torro, subtitles, wow-isms out the ying. The Descent a gang of hard 7's go caving and find creatures that be craving.
Here is a list of OG horror that may or may not be in their age bracket:
The Exorcist
The Shining
Salem's Lot
A Nightmare on Elm Street
Suspiria
Dawn of the Dead, the original ya dope!
The Silence of the Lambs
All to be watched in the dark and without skepticism or machismo.