I think is all you can say is along the lines of ..
After the game had ended, but whilst I was still on the field of play I became aware of two players having been involved in a physical confrontation where they were rolling around on the floor grappling. I didn't see it start, and wasn't able to ascertain names as both players were surrounded by parents and removed from the scene.
Were there actually any punches thrown that you saw, as if so that would need to be included. For future reference I would recommend approaching the team coaches, advise them that you would be reporting the office and ask them for the identity of the players. If either of them refuse that then makes it easier for the discipline department to raise a charge.