re: Fellow coach lost his mind at practice yesterday. What would you do?

If it's as severe as you described, the situation goes above coaching, football, hurt feelings, and "snitching"...being in the position you're in you're a mandated reporter of any child abuse and neglect...decking a kid definitely falls in line with the definition of child abuse.

You have a duty to report harsh behavior as a coach, and an even bigger duty to report child abuse as a mandated reporter. If you don't, I don't think you know just how much trouble you can get into.