I'd be checking to see if you actually did sign any waivers like that as it sounds a bit strange to me that they would not have insurance to cover situations exactly like that. I know when I was working, we could never have events on the weekend because our centre was only covered during normal working day hours. It's entirely possible that he WAS being closely supervised btw, these things can happen in a split second, right in front of you. Only takes a slip of the foot or a trip while running. But either way, I think there has to be cover of some kind. Perhaps the letter is a standard thing, advised by lawyers to send in the hope that no-one takes it further?