I'm saying 40k per year per kid per year. That's not a magic number, but it's something like where if you have 2-3 kids, one parent doesn't need to work because their income is replaced by a child subsidy. Alternatively, if both parents work, daycare is paid for, along with the other expenses of having a kid. Now, that does mean taxes have to go up a lot. Maybe you don't need such a high number. You could play around with it and see what works.

On the other hand, prospective parents have to believe that the money will keep flowing, or the incentive isn't as strong. In that case, maybe something like giving new parents a house is a better bet because they'd own that regardless of what future governments do.