Good afternoon again:
I'm sorry my answer wasn't what you were looking for. I'll be happy to address this aspect.
I think as a mother
, you already have the right answer. You want to protect your children while not depriving them of their father's involvement in their lives. This is in the best interest of your child.
One of the best ways to do this is to plan activities where the father can come be with his children, but they do not have to go to his girlfriend's house. If you are proactive about reminding him all the time that your goal is to not evict him from his children's lives and help him plan things that do not involve them spending the night there, he can have no cause for complaint.
Frankly, it's not in their best interest to be around this person, especially when there's not even a step parent relationship there.
Most Judges request that parents are cautious in introducing children to new girlfriend/boyfriends. It would be in the best interest of your children for you to sit down with your ex (sans girlfriend) and work out some ideas for sharing time with the children and some safe ways to do so.
Unfortunately, not everybody is capable of co-parenting on that level, which is why I encourage you to seek legal counsel if he continues to be unable to co-parent with you. If he freely admits that his girlfriend is crazy, but cannot see why that concerns you, there are some serious judgment issues there, and you would be acting in your children's best interest to limit their time with him.
Please feel free to ask any follow up or clarification questions you may have.