I think they should definitely at least be 10. When there are little 7 year olds there, it kind of freaks me out to be exposed to that kind of stuff. They might become desensitized at such a young age. A parent should definitely be present before 12, or at least the first few plays to make sure it is okay.
I hope these parents do their research on why the games are rated M. If my kid asked for one of those M games, I'd say no way.