Sadly you can't make adult children do what you think is best for them, no matter how hard you try, and you will probably alienate them in the process. DD kicked out SiL after two affairs and leaving her very heavily in debt. You long to tell her he is a waste of space and she is far better off without him, but if they come to an amicable agreement, you will be shut out.
DD sees her ex several times a week, which is good for the children. They spend Christmases together, sometimes he joins her on her holidays with friends for a few days. He has got what he always wanted, no responsibilities, the part time family he can see when he wishes on his terms. DD always said when they split up that she wanted the sort of Prince Andrew/Sarah Ferguson arrangements, and I suppose she has got it, but it does mean she can never move on, though of course he is seeing other people.