The kid and the mother deserved it. It's not rocket science, the kid will understand that bumping into people for no reason can get your shit kicked in. This is where all morality and etiquette has evolved from - if you don't play nice, shit will get real sooner or later.
I don't get why people are complaining about this arc. It's very entertaining. There's only so much "meaningful" plots that one can write out before the series eventually turns into a soap opera where every week someone is murdered or married or both.