This absolutely broke my heart, for you and for the kids.
If he shipped you off, maybe he will be willing to ship them off too. Try talking to him, explain all the benefits for him and how the children would benefit, although I'm not sure he'd care about the children's benefit. I would videotape them talking and how they behave so that someone can witness what's wrong with the kids. I also agree with calling the school and having the counselors talk to them, and tell the kids in advance not to be afraid and to tell the counselors the truth. Maybe see if you can be there when they talk to the counselor, or see if CPS will come to your house to observe the kids behavior when they're with you.
These kids are in hell; you have to do everything you can to help them. Remember that when things seem bad to us, they seem like the end of the world depressing and extremely frightening to a child. I remember those feelings all too well from childhood... and if my life isn't the perfect example of messed up, I don't know what is. Get those kids out of there, legally, but try to find a loophole because the system is terrible for helping children.
Always check to see if they have marks on their bodies when they are with you. If so take pictures, and call the police immediately. Please keep us updated.