Children of Men is an intense, powerful, and mind-blowing film...it was amazing! Everyone should see it, it's a masterpiece, no doubt about it.
As far as content goes, parents should definitely watch it before their kids do, but it's not nearly as bad as CSM makes it seem. The violence is intense, but realistic, and none of the violent parts are just tacked on there for the sake of it, they actually add to the story.