Starting at this from two viewpoints
a) having children is a lifestyle choice, not a necessity. In much the same way, my dog is a lifestyle choice, not a necessity. The government currently asks some taxpayers to subsidise the expensive lifestyle choices of others (but I don't see my dog getting 30 hours free daycare)
b) the world as a whole is overpopulated. Should the UK find itself short of labour in the future, immigration will provide a source of educated healthy adults in the required numbers.
AIBU to think that anyone who does not have children when they reach the menopause (or equivalent for men, to be clawed back if they subsequently have their first child unusually late in life) should be given a lump sum in recognition of the fact that they have not burdened the state. This could perhaps include
- child benefit, equivalent to 2 children for 18 years = £32198.40 at current rates
- two uncomplicated births on the NHS = £5580 (Guardian figures)
- putting two children through state education age 4-18 = £154,000 (IFS figures; add more for your free nursery hours and any higher education)
(Full sum to be paid to married couples, half each to single people)
I'd also pay those who have only adopted or foster children (as they not responsible for bringing the children into the world) or only children who have died before their 18th birthday (because otherwise it seems a bit harsh).
I'd oppose removing child benefit and the like from those who have children purely because of the impact on child poverty, which impacts those who did not choose to be in that situation and has all sorts of counterproductive effects on things like educational attainment and health outcomes.
However, in short, why are people who choose not to, or who are unable to, have children, asked to pay for those who do choose to have children? Why shouldn't they get something in recognition of how much money they've saved the state? It might also concentrate the minds of those considering having children on a whim / because that's what people do / just because. With the above, everyone ends up getting the same amount sooner or later, it's just that some have it spread out over time and others get it as a lump sum; it would be fairer than the current system.