I think you are doing beautifully with your kids, really amazingly well. You're one of the best at not putting your kids in the middle right off the bat that I've seen in these parts.
It is only in trying to struggle with STBX over this stuff that you'll risk sliding into putting the kids in the middle more than is necessary. They are sensitive little things. S KNOWS you feel validated when he talks poorly about GF even if you don't say so. S KNOWS it comforts you in some way. S WANTS you to feel good. Part of this is unavoidable. But you can avoid letting it blow up into a much bigger thing by not letting some of his caretaking of you become something that has power in terms of how it affects his situation. It shouldn't. The deal is done. Visitation is settled. Take it off his table.
And, FWIW, I wholeheartedly agree that STBX is an A$$ who is handling this in a pretty boneheaded and selfish way. Sorry he is that way. But there is a LOT you can do — finding the support group for the kids, finding a C, being open so they can talk to you (which you obviously are), being supportive of their R with their father (which you obviously are), and so on.
You have many ways in which you can help your children. You are smart enough to already be finding them and caring enough to already be acting on them. The one thing you can't do is manage their R with their father or make his choices for him, nor should you try.
