I think you only have to look at the difference between the way welfare reform and child maintenance reform are handled to see the huge disparity here.
Welfare reform is seen as saving the public money, and there are plenty of policies there that risk placing families, including those with children, in poverty. A lone parent of job seeker's allowance, for example, is at risk of being sanctioned which directly affects the ability to provide for their children.
In contrast, child maintenance reform is seen as a "private" issue that only affects each individual family, and one of its central goals seems to be to avoid placing the paying parent in poverty even if they are single with no other dependents - WHY is this, when the government through welfare reform is willingly placing families with children into poverty?
I think it's short-sighted for child maintenance not to be considered a public spending issue - if my XP had for example paid even £50 a week (ie, 25% of full time NMW as we have three chilren), rather than the actual amount he paid which was £5, then I might have been able to return to work earlier, or to offer my children more enriching or extra-curricular activities (and given that poor white boys have the worst educational outcomes that might have been priceless), or able to provide any number of opportunities which in the long term would save public money either through my own increased ability to work or through increasing my children's attainment (tbh it would just have been nice to be able to replace worn out clothes as soon as required but that's by the by)
So if we take a leaf from welfare reform, the first thing I would suggest is that NRPs are assumed as a minimum to be capable of working full time for NMW and at a minimum should be assessed as if that's what they're earning. If they have caring responsibilities or health issues, these can be assessed in the same way as for benefit claimants. If they're genuinely not working full time, they can attend the job centre and be forced to look for work in the same way the JSA/Universal Credit recipients are. If they don't attend, the debt racks up each week that it's not paid, and it's enforced and it's taken from wages or bank accounts and it is treated like a debt to the government.
(Personally I think what should happen is that the government pays the assessed amount to the PWC and then chases the NRP for the debt like a council tax debt to ensure the children get the benefit of the money continuously and immediately, but that's never going to happen.)
If I can be expected to work increasing hours to support my children as they get older, as a lone parent of 3 with literally 100% of the caring responsibilities, then why is their father not also expected to be working his backside off to provide for them when he has none of those restrictions?
It's about that central idea - you should provide for your children. If government policy is to push and push welfare reform (including tax credits) with the expectation of people working fulltime unless they've been given an exemption, then it should be exactly the same for NRPs.