Clothesline

So, I am just wondering, if your child is playing with other children in a play area and you see some other kid come up and try clothesline your child, do you just sit there or do you run up to the child and yell at the child. Keep in mind that your don't know the other child and you don't really yell at the child, just tell him to get his hands off your child. And then you realize the Mom/Grandmother is just sitting there just watching and not doing anything. I mean seriously, the little boy would have taken Gwen down had I not stepped in. It is not the first time I have fussed at another child, but I figure if my child is not able to talk for herself and an older child is not paying attention then it is within my rights to fuss at them, especially if injury could happen. (I have been known to tell older children to apologize to younger children when they knock them down, even if Gwen is not the one who was hit) I know read that you are not supposed to not get involved and let the children figure it out for themselves, but I am of the impression that concerns when children of the same age are playing together and they start arguing and such, not when they can't talk and someone comes out of nowhere and tries to beat them up. What do you think?