Sending some support here op. My dh was in a similar position. He did maintain contact with his kids but it was hard and their relationship is distant now.
His kids are now similar ages to your dh's so I also hope that the attitudes of the family court have changed towards parental alienation and disrupting contact.
But short of jailing the parent or moving the kids to live with the other parent I don't know what the courts can do even now.
There was never any suggestion of jail for the mum, I don't think dh would have wanted that anyway, and when he applied to court to have the kids live with him and he would facilitate contact with their mum the judgement was that they were better off with their mum because that was their established home and they barely spent time with dh. (Because their mum thwarted it at every turn)
I think sometimes you come up against a particularly difficult parent who just doesn't put the kids first. That can be a mum or a dad.
From what I saw, putting the kids first means protecting them as much as possible from the intricacies of negotiations, not criticising their other parent and not getting in the way of whatever relationship your kids want to form with their other parent and that parent's family, whatever shape that goes on to take.
And if both parties are not willing to do that then it really is tough on the kids.
I'd be interested to know if people do think this situation is dealt with better nowadays.