ISIS trains kids that young to fire AK-47's in combat, and as so far as I am aware they haven't had many safety issues...
I'm sure there's a point you're going for here but I don't think that citing ISIS as an example children can handle guns is a good way to do it.
and I'm not sure the children have had many mental health issues about shooting infidels in the head, unless you classify children who relish shooting infidels in the head as some sort of problem.
I would think children growing up in a terrorist organization are a lot different, mentally, than children growing up in American suburbia when it comes to the value of life.
You're utterly insane if you think a perfectly normal 9-year-old girl who has grown up in an ordinary American home isn't going to suffer any mental issues after blowing a hole through the top of a man who was standing right next to her.