My 9yo daughter was fine, but she loves scary movies, especially vampire ones.
She says the zombies and things didn't scare her at all, it was the ghosts suddenly popping up on the screen that made her jump (me too, lol), but she likes that kind of stuff.
The movie does have some fairly intense scenes. I can see where some kids may very well be frightened. I guess I'd say if your kid freaks out at things like the Fantasmic! or the 3D shows at WDW, I'd not take them to see HM. The movie is WAY more scary than the ride.
We enjoyed the movie. The story line wasn't anything like we expected, but it was okay.
It was fun looking for the things from the ride, and this movie is filled with them.
Be sure to stick around through the entire credits. There's something to see at the VERY END.