Yes, it was a kids' fight and I witnessed them as a boy. The difference is that, given his injuries, this boy in all likelihood was knocked to the ground by a punch or punches (the broken nose resulting), whereupon he was kicked repeatedly in the head. This is speculative but it is based on first-hand observation of the MO of thugs such as these. It's what they do.
If they kicked him in the head while he was down... then I would have to agree with you then. That’s crossing the line into assault and possibly attempted murder.