What habits are they picking up?

I guess they might stay there the night or this wouldn't be so much of a problem. Suggest to your ex that they stay a minimum amount of him at his quarters and take them places to spend quality time together.

If they are learning some pretty bad stuff and aren't being given a suitable environment to stay in, then you should bring it up to a court to have things changed a bit. Your children's best interest is at stake here, and your ex isn't caring to try hard enough to give them what they need.