If that grandmother mouthed off to me because she refused to stop the kid from kicking my seat back, it would have called over the FA, and in front of grandma asked the FA to have a police officer meet the airplane when it lands because I want to file a disorderly conduct complaint against grandma.
I highly doubt the FA would go ahead and ask for a police officer, but maybe the threat alone would have been enough to control her grandchild.
Mr. Elliott