Mother’s physical discipline leaves witness unsettled

DEAR ABBY: I was shopping recently and encountered a mother who had her two young children in her cart. One of them was making a loud noise. The mother slapped her hard across the face, yelling, “You’re not getting what you want, so BE QUIET!” I felt I should say something, but I didn’t want the situation to escalate. What would an appropriate response be that would show empathy but would also convey that physical abuse is unacceptable?