You have to stop sleeping with him. This would be the first step. I think it is very important for the kids sake that you maintain a friendly relationship with their father. As posted above, meeting in a public place or something like that is a good start.
Let me ask you this. Is there a chance you guys might want to get back together? I was just wondering. if you still have feelings for eachother and sleeping together is part of the equation I'm wondering if he has feelings for you as well.