For the poorest in the world, people with no social safety net, children are your 401k, pension, and social security system. Can't have children? Enjoy starving to death in your golden years. Conversely, many children means a cushy retirement. In ancient times your children were your literal property for your whole life, and that's still generally the idea in poor countries.

Civilized countries decided that forcing poor/elderly childless people to starve wasn't acceptable, so we have these support systems which make childrearing optional. Furthermore, everyone decided to trade single income families for doubling national productivity.

To stop overthinking it and make things simple, all you need to do is get rid of social security and pensions. This immediately changes the math for everyone considering children. Firmly gripping the third rail of politics is super straightforward!