I've been through this too. Best thing is to be neutral. DO NOT cover for him, or make excuses, or put any kind of a spin on it. Just hold their hand through it.

DO NOT trash talk him, fight with him in front of them (even on the phone), be outrage on their behalf, even if you feel it. Don't let him make you a prop or a go between. If your child wants contact try to honestly facilitate without judgement and let them see you do so. Let him speak for himself with the child(ren). When the kid(s) ask if Dad is ...whatever. You don't know. Let them reach out and ask him and let him take the hit. Do not chastise them for being properly hurt and angry if they are. Just hold them through it.

Kids aren't stupid. Just give them honesty and effort and they'll figure it out and adapt.

Honestly, when I adopted the stance of neutrality, it got better. He faded away. I severed his parental rights based on abandonment and we moved on.