I'm one of those who is the main carer (only carer he isn't allowed to have contact as he's too abusive even for the courts), and twatface messes around paying less than he should or late and we've had years of him paying nothing. Whilst he lived in a massive house and drove the latest luxury cars. I know because he would make it a point to sloooowly drive past me and my little DC when we would be waiting at the bus stop (top bloke that, can't think why I left him).
At no point in time did I think well his new wife (who was also OW) should pay for my DC. And she would also make a point of walking up and down her drive way holding her child to show us what she had if we were ever walking past her house or waiting for a bus near her house (I used to avoid using that bus stop and walk to the further away one).
My DC are my responsibility, ex is missing out so much from mistreating his own DC. Meanwhile I have amazing gorgeous, super clever, accomplished young people in my life whom I love the bones of.
Ex owes my DC a lot of money, his wife owes us nothing. I hope the money he is withholding from his children brings him nothing but abject misery.
I really don't get the, well his wife/partner owes me money. She doesn't she really doesn't. I bet all those saying pay her the maintenance, would be up in arms if OP called the ex the birth mother, or wanted the DSC to call her mum, or if OP tried to attend parents meetings and have a say in her schooling, medical decisions etc.
OP is a responsible adult in the DSC's life, she is providing love and support for the DC, she is not financially responsible for the child.
And in this case the father is trying his hardest to ensure he is able to contribute towards his children financially any way possible.