Never let your kids feel you gave up on them - no matter how much they push you away. Let the lawyers do their jobs. Take your son to do fun things together without GF for the time being when you have him. Maybe sometimes things D would have also enjoyed. And I'm not saying to bribe them, but D seeing S doing fun things she might have liked too may make her start to rethink things or feel some resentment about your Ex keeping her away. Communicate with her as best you can without being pushy. (My current BF went through a difficult divorce when his three girls were college/high school ages. The girls all blamed him - no, he didn't cheat on his wife, and to hear him tell it, she's an alcoholic but I really only have his side of events and I know he can also be difficult to live with. End result is, he feels hurt by them pushing him away and so he pretty much gave up on contacting them. I'm sure this only fed the negative cycle and he has had minimal contact with them even though he has a fatal disease. I can't help but think that if he had pushed through and continued to put their feelings first and kept letting them know he loved them even when they didn't respond, he would probably have a relationship with them by now.)
Also - and this is just a thought, not something to act on right now, but - do you have pets? If not - might you think about getting a puppy? S might be a LOT more excited about coming to visit if there's a puppy involved. Now, definitely DON'T get a dog if they're not your thing and it's not something you would have done anyway. And don't do it right now because a newborn AND a puppy is just too much. But I was just thinking about how my family got a puppy when I was a baby - I suspect it was a consolation prize to my older brother for having to give up some attention to his baby sister.
