I think no overnights has to be a given. His options could be going out to lunch or something on the weekends wherein you drop them off and pick them up at the venue? Or supervised visits only? It just cannot be worth your children's safety to not put your foot down and stand up for them. Imagine how uncomfortable it must be for your kids to try to stand up to your AH. Until they can find that courage, you need to be the parent and do it for them.
CPS is another good option, I think. They can help to make things very clear, legally, to your husband and kids.
I know they don't want you to talk to him about it, but it's really important to not turn a blind eye. Demonstrate to your kids that we do not enable people by keeping their secrets, and we do not allow someone to put us in danger, ever. Be the example of strength these kids need. It's hard for kids to stand up to a parent, and even harder to be in the middle of disagreeing parents. As much as we want to have them equipped with the knowledge to protect themselves (I.e. Calling you for a pick up), it's more important that they not be in the situation at all. That lies in the role of the protector (YOU)!