I'm stepmother to two, now aged 23 and 18, I've been married to their Father for 13 years. I was NOT the reason for the divorce, didn't know him then.
The 18 y.o. is just doing A'levels, and about to go to Uni (as far as we know, she doesn't actually have much contact with us, her choice).
I've always accepted that dh had responsibility to pay for his children, and have never complained or commented. When his eldest was 18 and left home, dh reduced his maintenance payments to X wife by 1/3, so has been paying 180 pounds per month.
Until a couple of years ago we saw the kids every week, but recognised that as they grew up they had things they wanted to do, so made it an informal arrangement, and told them they can see us whenever they like, just ring.
DSD has basically stopped any contact with us. She has never shown any affection or regard for her father, NEVER bought him a birthday card, or any sort of present. There isn't any animosity, just a general 'can't be bothered'ness about it. If dh phones or texts her, she doesn't chat really, or doesn't reply.
I have suggested that when she finishes school, he needs to change his approach to maintenance (assuming she stays in education. If she doesn't, maintenance stops anyway). I suggested that he pays it to her, not her mother, at £150 per month.
I think this is more than many students will get, and as her mother now has a very good job (I reckon earning as much as dh does, if not more) she can contribute too.
I'm not intending to be mean, but I do feel aggrieved that she makes no effort to contact her father, or her little step-brother, who misses her a great deal, and often asks why we don't see her any more. She didn't acknowledge his 6th birthday, BTW.