Heya Buffet...SO glad the bar's still open! I'll have a glass of pink champagne thanks - and I think you need one too (but just a little one *hugs*)
I know how angry you are about this whole debacle but in all honesty (and I KNOW you're going to hate me saying this but it's the truth) It's NOT the OM's fault your S2 fell and got hurt. I suspect that you'd still be mad if it happened elsewhere, but you wouldn't be as vitriolic about it...no?
Your XW should have let you know that she wasn't taking him to daycare and yes, that was stupid of her not to, but that's where the fault ends. Kids fall over and get hurt, it happens - just don't let this blow up into something that is avoidable.