The reason that it doesn't work is because governments don't pay enough. You can even pay enough to cover the entire cost of having the child (food, childcare) and it still won't be worth it for the parents, economically speaking, because of the opportunity cost associated with maternity. Of course, if governments start paying one hundred, or two hundred thousand dollars per child (or less if combining child payments with a childlessness tax) then it starts becoming an economic incentive - if you aren't having children by X age then you start getting left behind by your peers.