OP, I completely get where you're coming from.
I have a 50/50 arrangement for my two girls. I'm a higher earner, and in the divorce I gave ex every penny of the equity that had built up over the whole course of the marriage, so that she could buy a house virtually mortgage free. I stayed in the FMH to give the kids stability, but as a result have a huge mortgage.
Technically, no maintenance is payable - but I pay some anyway, because I earn more. I pay for all of the school uniforms, school trips, and other major expenditure. I just paid out over £300 on uniform for the new school year, with my youngest starting secondary and my eldest needing quite a few new bits of uniform. I've just paid £350 for a WWI battlefields school trip for my eldest. My ex pays not a damn thing to any of these costs. She gets the child benefit and tax credits, but all she really has to pay for are some clothing costs for her house, and food & utility costs.
Despite that, she is constantly complaining to the kids that she doesn't have enough money. They've come to me and told me about how she refuses to buy them stuff for school because she has no money, and then immediately goes and buys herself another pair of shoes.
She's remarried, and so has a second income in the household. The kids and I do have a comfortable lifestyle - a nice home, good holidays, days out etc. But I work my ass off to provide that - I have a demanding job, that I manage to balance with being there for the kids, and I've even taken to renting the spare room out on AirBnB when the kids aren't with me to make a bit of extra cash. Every penny I have goes on providing opportunities and experiences for the kids that I never had when I was a child.
Meanwhile, my ex just complains and expects to be kept without having to make any effort herself. Recently, she's even started refusing to give them enough money for their school lunches on the days they're with her, and telling them to get the extra from me.
So I get why you're annoyed. But I look at it this way - everything I'm doing, I'm doing for my kids. I could get angry. I could play rougher, and cut what I pay her. Or I could make more of a fuss about her contributing. But I don't. Because all that would do is cause problems - I don't want the girls worrying about money tensions between their parents, and being scared to ask for things that they need because of the problems that could cause. I'd rather just suck it up, and pay. Because it's best for them.
So that's my advice to you. By all means, ask for contributions to stuff. But if he isnt co-operating, don't escalate it to a point where the kids become aware and start second guessing whether they should mention that next school trip, or the stuff they need.