I do agree, the first two are not okay at all. I don't know your relation to the children at question, but if you are in a position to do something about it, you need to.

As for the third, that is okay to me...but I am concerned if that child is in the same group or relation as the children in the first two, if that makes sense. I would be concerned for sexual abuse, if that's the case. If the child simply knows how to masturbate, I don't see that as a problem. That's not unusual to figure out that touching "down there" is pleasurable. If he seems more knowledgeable than someone who would just accidentally stumble upon this would be...then I may be concerned.