My DD went to a Halloween party at her school friend's and I've found out they all watched Insidious ... I am upset about it. I would never allow my child to watch an 18+ horror movie (or play an 18+ violent video game).
I wouldn't expose someone's else's child to psychological horror, death and violence without definite permission from their parent... I feel upset about it.

I've looked after my daughter carefully with regards to content in books and films, as I was sadly exposed to adult themes far too young
P.S. DH and I have watched the film ourselves and it is disturbing on many levels.