In any game, you can kill any person above 18. These people may be completely innocent, but it does not matter, you can kill them.
You should be allowed to kill children, just like adults. It doesn't mean you are going to start killing children, as if that was the case, then you would go around killing other people.
Just because a person can be a child though, doesn't necessarily mean that they are innocent. Just like any adult, it is when the person is mature enough, or have some "loss of innocence" moment that really affects a person.
In reality, a person who is around so much death will "age" quicker then other people who are secluded.