I don’t believe I missed the boat. The kids are not in the right, but JoJo could have handled it better and shown the kids the right way to behave. He went to sit an an emppty seat at a table of 10-12 year olds. He didn’t ask if the seat was unoccupied ( for all he knew an adult was sitting there ) and was rather nasty in his response. I assume that if he went to wash or get some food, he would have no objection if another adult took the seat.
Too many parents bring there kids to shul and let them run around. The best example I’ve ever seen of a lack of derech eretz training by an adult happened to me 3 years ago on a Shabbos morning. I was standing while davening and suddenly felt something kicking my legs from behind. I turned and saw a 3-year old sitting in my seat. I looked at the father who proudly exlaimed he was teaching his son makom kavuah ( they had come in at the end of Pseukai Dezimrah) and I was sitting in his son’s seat!!