Oh blessed, no advice to give, just empathy as my DP is in exactly the same position. He so regrets sorting out financial side of divorce (during which amicable every other weekend with DC, who are 100 miles away so no possibility of midweek) and only then trying to formalise contact with DC when exW became increasingly difficult.
I think some women just use the DC to get at exH and lose sight of the need to respect that DC's relationship with their father is not one they should meddle with after family breakdown, if there is no reason to. It is spiteful and DC suffer in long run. DP unable to fathom where animosity has come from as he was the one who was rather suddenly asked to leave, quickly followed by exW initiation of divorce proceedings. Probably relates to exW's father deserting family when she was young, and this is her way of getting back at father. Who knows?
He's tried to appeal to exW's better nature with conciliatory letters etc (she will not answer phone to him, and he now only has address for her and DC, in addition to DC mob numbers, which they often fail to answer) to no avail, and is increasingly frustrated by her behaviour (as well as lack of time with DC and hence fragmentation of his relationship with them) - no one at home when he arrives to collect DC as arranged (and once police called), DCs presents 'never arriving', texts from DC cancelling arrangements when he is en route, and when he has seen then (about 6 quick lunches this year, no week in the summer) he's always made to wait a couple of hours at station before they arrive. He's suggested to DC about doing things, they've been excited, he's bought tickets etc., all seems fine, then receives 1 line letter 'they will not be going to x,y,z). At vast expense and anguish, he's resorted to trying to get a court order with final hearing next month; still not terribly hopeful as DC now 10-13, and thinks it will all hang on the judge he gets (in preliminary trials there seems to have been no consistency in the direction of things). Statements from exW simply state 'it interferes with DCs' social life', so like in your situation, lack of contact is made to appear the wishes of the DC, who DP is sure have been rather manipulated to tow the line with their mother's wishes. It's very important to DP that DC will know that he has done everything to try and see them frequently.
Wish I could be more positive, and hope your DP will have a happier outcome. I can only imagine how very sad it makes him.