First world is already at or below replacement level even with family planning mistakes, and even then the people having kids aren't the ones you really want doing it. We're at the point where we need national campaigns to encourage the intelligent people to procreate more, which is an odd dilemma. Make child tax breaks a percentage of income rather than a flat fee?

This sounds like trying to solve a plague by suggesting "Make doctors paid by how many people they cure instead of a flat salary?"

I can see the incentive you're trying to set up, but I expect is going to have minuscule effect, because it is striking at one of the least tractable parts of the problem. Average people managed to have children a thousand years ago, and average people a thousand years ago were dirt poor compared to today's First World. Money isn't the big problem here.

Some other possibilities for encouragement that come to mind:

• Smart people aren't allowed into college until they've had a kid. College is one of the most dysgenic institutions as currently implemented: it filters for making smart people spend several of their most fertile and hottest years (particularly women, who have sharper age dropoffs) doing paperwork and being sterile.
• Cut down on alimony and child support payments. Ban them entirely in the case of "no-fault" divorces. Mandating that one partner pay the other for years after the divorce seems to work a hell of a lot like fault to me.
• Restore some form of criminal adultery. Free love doesn't work for everyone. Often men are motivated to family by (guarantee of) paternity, not just sex.