If they have the freedom to choose otherwise, they will. The global fertility "crisis" is simply individuals exercising their choices. Kids are really expensive, and if you want people to willingly have them outside of accidents, you're going to need to pay them a lot of money.
Rising kids also has a time, effort and opportunity costs which are not easily offset with money. I don’t think there’s a way to frame modern parenting in a way where it „pays off” in the same sense as it did in the past. As of now, it’s essentially a hobby.
The problem is that there will be far too many people wanting that job, so you have to filter somehow, and that's basically eugenics which isn't the most fun, so I guess you'd have to have a lottery and deal with the fact that like 10% of the population will constantly riot about someone who "doesn't deserve it" getting paid to raise kids.