Attorneys, and other legal experts, will probably tell you to just stand back and take numbers. That being said, it's very difficult for many of us, especially those who are also dads, youth league coaches, etc., to just stand back and watch kids wallop each other.
I've heard stories, don't know if they're myths, or not, about officials who hold one of the players in an attempt to stop the fight, but end up giving the other player a clean shot at his subdued opponent.
I don't think that there's an easy answer here. Reminds me of situations where there is no trainer available and I'm the only adult around who knows a little first aid. Do I stand and watch the injured player suffer, or do I step in and offer assistance? Damned if you do. Damned if you don't.