Man, I'm not really a good judge for that, Mike. Since I don't have kids I just don't really have a good grasp on what a kid can handle.
This is certainly what I'd call a sci-fi movie as opposed to a horror movie. There's only one kind of suspenseful moment (in a subway tunnel) and only a couple "jump in your seat" moments. There's some blood, but I wouldn't call it gory by any means.
There's a reference to sex, but it's just a reference... talking about 2 people that have slept together, and a few "shit"s.
I hope I've helped some - sorry I couldnt help more.
EDIT: On second thought, I'm not really sure how much an 8 year old would really enjoy the movie. This this is shot entirely first person, like Blair Witch, so there's a lot of questions that go unanswered. It might be over an 8 year old's head just in that regard.