The problem with subsidies is society does not benefit from kids simply existing. Society benefits from well raised kids. Kids not raised well end up costing society.

As “well raised” is not possible to measure, this is not a feasible solution.

One work around has been to bet that those with earned incomes are likelier to raise their kids well, so the subsidies can be non refundable earned income tax credits.

But I doubt sufficient people have sufficient tax liabilities that offset the costs of the minimum quality of life many people require for their kids and them to have.