It's incredible how difficult bureaucracy makes it. Surely it's enough that a parent's name is on a child's birth certificate. That makes them responsible for a minimum of half the cost of feeding, clothing and providing housing for that child until they are 18 and it should be index linked. I can't see that this is in any way an over simplification. If the parent with custody of the child/children re-marries or co-habits, so that two adults are maintaining the home, part of the money from the other parent could be held in trust for each child until they come of age. The rest is still needed to pay half the cost of clothes, shoes, food etc. This would apply whichever parent has custody of the child/children. Obviously, where there is joint or shared custody, adjustments would be made. Simple! ?
There used to be a subject at school called home economics but I think it might've been a fanciful name for what we called cookery. Let's drop RE and introduce a compulsory lesson called real home economics so young people have some inkling of how much it costs to live independently, responsibly and bring up a family. They would learn about the impact of smoking, drinking, eating out, take away food, fancy holidays, phone contracts, satellite TV, running a car, rent, mortgage, council tax, water rates etc. on their income before they ever dream of becoming parents. That might help to prepare them for the reality of adult life.
Mind you, I think GN would fold! Maybe I'm going too far with this. There'd be very few grandchildren around! ??