As we mentioned earlier, airplanes are lawless places where boundaries and personal space don’t exist. Nobody likes sitting in economy, but we need to make the best out of the worst. This kid decided to lighten the mood by playing tag with an immobile passenger behind them.
Since it’s a kid, this guy couldn’t just scream at them for touching them without reason. In fact, the OP found humor in the situation. He totally understood that kids have a problem staying still, and if playing a game of immobile tag could keep the kid from screaming their head off, so be it.