I wouldn't give it a second thought. If they bring it up tell them that from this day forward if you see their kid being physical or inapropriate with your mother that you not only will say something but will physically remove her off of her if that is what it takes. Don't back down. Tell them you feel very comfortable with what you did and you would do that and more if it should happen again. When the kid is around your mother and you are there you don't care who her parents are or what their opinion is. Mama comes first whether it's a neice, a neighbor, or a nun.