My dp has dss 50% of the time. When he worked 9-5 he paid for most of dss' childcare - his ex had the rest paid from tax credits and then she paid for most clothes etc. as she got the tax credits and child benefit. When her partner moved in and she lost the tax credits then she started trying to get more money out of him (which he didn't have).
Recently, he has gone self-employed and after negotiating with her they decided to share the child benefit and then split costs 50:50 on the proviso that they would discuss any purchases that needed to be made first.
Predictably, it is getting a bit petty. One issue is that dp collects dss from school every day which is in a neighbouring town. So if he is going back to his mum's dp will be driving backwards and forwards from neighboring town 3 times in a day so is constantly filling his car with petrol. It also means he is taking time out of the business which he has to make up at other times. He is happy to do this but we felt it was a bit of a drain on our already tight finances. So they agreed she would pay a small amount of petrol money each week but she is now looking for any reason not to pay it. She has asked dp to pay half the cost of a passport because she is taking dss abroad (no chance of us going anywhere abroad anytime soon) and half the cost the cost of a bike despite him having a bike in our shed. She said he needed new vests and dp pointed out we have loads here and we'd send them over but she bought some anyway and is now saying dp needs to send her the £3 or she won't be sending half the child benefit. Our finances are joint so it affects all of us.
I am starting to resent it tbh. My dcs' dad pays nothing for them so I have to be frugal. Nearly all ds's clothes are second hand (which is fine, he has some nice stuff) but I only ever buy stuff for did if she really needs it. DSS has mountains on clothes and mine just have what they need.
I think my situation probably clouds my judgment. I've had to manage without any financial support from my ex so I don't have much sympathy for her.