It depends on the harm that you witness. Your question does imply your awareness that parents are not/may not be intellectually (or even morally) competent.
In no Western society that I can name are parents omnipotent owners of their children. Parents may even lose custody of their children. If you know that parents are doing physical harm to children, you have a social obligation to try to do something for those children.
Even though we may turn a blind eye, we do have a social obligation to all children. Human anthropological history reflects this.
Although intellectual harm tends to be seen as sunken cost (and possibly "correctable"), social harm has intolerable consequences.
No one is an omnipotent owner of anyone, not even the community, not even the government.
If you strip away conventions bound in law, governance the idea of community, and go to basics: if I see something going on that troubles me, I should take whatever action I think is appropriate. No 'community', no man made laws required.