2 parents - no longer together
Kids live with mum, and occasionally see dad, when dad wants to - usually in school holidays.
Mum has more money generally than dad. Not massively more, but enough to get by. Mum does not claim maintenance and pays for everything for dc. Dad occasionally gives money for specific things, like school trips. Mum does not ask for this but accepts when it is offered.
When dad does see dc he usually asks mum for money for travel, food or activities while dc are with him.
Mum does not believe that she should have to contribute to his costs while he has the children. Every time he mucks her about, saying if she doesn't give money he can't have dc after all (having told the dc about the plans and they have got excited about it). He usually ends up taking them anyway, but not until he has got mum thoroughly upset, often for weeks beforehand.
WIBU to tell mum to just not engage, say he can have dc on dates requested but she's not contributing and then not discussing it any more?
He guilt trips her, says he can't afford food and heating when dc aren't there because of the costs of having them. Says he's so generous because he contributes to some things here and there and so she should pay for xyz. Kids tell him mum has bought for eg. a new household item, and dad then says she can afford all these luxuries so she should pay xyz.
I expect it's obvious by now, but they split due to abuse - all sorts including financial.