Hi Nambie,
this must be a horrible time for you and your kids that's for sure. I think you have alot of courage for talking to him about this. Since your daughter is 11, I don't think he should be slapping her on the butt anymore. It's cute to do that when they're babies but not 11 yrs. When things are a little calmer have you thought about asking your children if there has ever been anything inappropriate in the past.
I think his reaction was very drastic and I don't think you should feel guilty. I'm sure your children are very upset. I hope everything works out for you and your children. .