My ex keeps calling and asking for me and our 2 kids to hang out with him and his g.f and her new baby....?

Under no cirstances should you subject your children to that environment or father (who has major issues) any longer than you absolutely have to. I have 4 boys and there is no way that if my ex were like yours, that I would let them go over any more than court ordered. As far as how to explain to him the visits are for him only, just come right out and say it. The first few times of letting him know exactly what the deal is, is very hard. After a few times it gets easier to maintain your control in this situation. You have to realize your the only protection those kids have. The courts can only do so much. Also there should be a designated place for him to see his child and that doesn't include hanging out with current girlfriends. If there's not a designated place maybe you should file to obtain a mutual place where he can come to see the kids. That would take pressure off of you having to explain not hanging out. Best of Luck.