I marvel that you are stuck on the child either being rude or letting it go. If my kid was there he would easily have come to me and asked me, as an adult, to say something to the man. If there were no adults to ask, i have no doubt he would have said something respectful, apologizing for speaking out of turn and kind. He wouldnt argue back or shhh an adult. Is your inability to see more respectful options a product of not having any on the tip of your tongue? How then do you educate yourbown children to have respect?
Menu