I'm with JohnBovi on this.
we have been in exactly the situation Petal describes - with school fees mounting up, and more children (mine and dh's) - still only toddlers at that point, so not an issue right then, but one that was waiting to happen.
we couldn't afford dss' school fees, but dh's ex would not let dh not pay. we moved heaven and earth to work that one out, including massively downsizing our house, which meant that dh & I had an unfinished loft room (and by unfinished I really mean unfinished - no windows, no heating, not even painted when we moved in there. just a boarded out loft, really!) while the children had the proper bedrooms.
we would have done similar to keep the younger children at school (ie if it happened now), so why wouldn't we do it for dss? he is still part of the family.
dh's ex hasn't gone on to have more children, so we don;t have that scenario, but if she had, then dh's maintenance would not hav decreased, and would still have increased. and once the maintenance leaves his bank account, it is no longerhis. it is for his ex to spend as she sees fit. for the good of the family (and that owuld have included more children if that is the way it had worked out).
dh may not like the decisions she has made - and she has, in our opinion, made loads of crappy decisions. but that is for her to work through with the children, when they work out what went on.
just as people oftne advise single mums to keep out of the relationship with a feckless (but not dangerously so) father - a give him enoough rope and he'll hang himself type situation, wrt crppy timekeeping/not turning up for contact/forgetting birthdays etc. the advise is to let the relationship be what it will be. and it is the same for us.
dh has doen all he can. he has provided for the children as he always intended to - school fees, now uni fees, all extras (hobbies etc), maintenance at above the csa rate, as well as spousal maintenance. (his ex works, btw, in a good job, with a good wage, so has also contributed)
the money has not always been spent on the children, but that is not dh's fault. and that fact does not mean he should not have paid it.
we have gone without ourselves (when I had dd1, she had a cot, one toy and 2 packets of babygros. nappies and muslins. that was it. literally. and this while dss was at one of the most expensive schools in the country. but what else does a baby need? really?), and we have lived at a standard that my stepchildren have turned their noses up at.
but we have done what we would do for our children - any of them.
equally, we have uprooted ourselves and moved around for dd1's schooling (she is severely disabled). I have undertaken large parts of her therapy myself, becasue we couldn't afford to pay anyone, as we were paying school fees. so be it. my stepchildren didn't ask for dd1 to be disabled and cost more - why shoudl they pay the price by having t be taken out of school?
I agree that a NRP should not be impoverished, if there is any other way. but if you would have done something for the children if you had stayed in the marriage (eg downsizing to keep in school, or accepting a lower standard of living to make sure the children don't go without), then you should do it when separated. the children are still your children, and they didn't ask for any of it.