I see what you mean, but it's simply doesn't work this way. Raising kids is so life changing that it has no money equivalent, like you can't buy another life. So if that mentioned self-interest is strictly materialist, one won't choose to have kids no matter how generous are welfare programs. We can find moral or practical reasons to support (or tax-relieve) those who does this work for the future of humanity, but it has nothing to do with low birthrate. You can't fix it by throwing money anywhere. It's a problem of ideals.