robin, How sure are you that the head would turn a blind eye and not tell the boys parents because the incident happened out of school time? I think if this had happened at my son's school, the head would have said something tothe parents. After all it was a school event (even if it was via the PTA), and took place on school premises.
Don't know your head, of course, but do think it would be worth talking to him first to see if he has informed the parents. All he has to say is yes, no or 'can't tell you that'. Then you can decide if it's worth taking further.
If you decide to talk to the parents, could you just say that there was some disturbance involving the boy and the head had to intervene. Say you didn't witness it, but if they want to find out more, why don't they speak to the head themselves?