In a nutshell, I have 4 DC. 3 have one father and one has another. XH (father of 1) gives me maintenance every Friday for DS which I usually put towards the weekly shop. He has a visit every Sunday. This week, XH had some errands to run so drop him off an hour early (he usually baths him and gets him ready for bed before bringing him home). As he was still dressed DS asked me if he could go to the shops for ice cream. XH hadn't left at this point. I said I hadn't got any ice cream money to which XH said "what happened to the money I gave you Friday?" I said I had been shopping and needed electric. XH replied "That money is for DS not for you to spend on yourself or the others"
All DC are clothed and have a roof over their heads which is out of my pocket as is the rest of the weekly shop and all the bills.
I don't think IABU to use his maintenance money this way. It's not like DC go without while I spend it on nights out or clothes for me, if anything I go without so they don't have to.