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.