Not sure how that would work. We are going to pay only those in a higher socioeconomic class to have children? That's going to be more than a little sticky.
By paying payers to have these children, a whole bunch then would be shifted into your ineligible bracket because a couple CTCs will push a lot of people into zero payer status. Never mind that you really stick it to folks who work/earn good money but have big families. (Hint hint)
Wouldnt making it simply non refundable be vastly cleaner (albeit much less effective as an inducement)?
Margins are a witch.
And that's why you don't want such a steep curve on either side of the line between payer/beneficiary.
And it's also why the tax code should contain no subsidies of any kind to any business or person for any reason. When you combine the subsidies and the taxation, nobody really knows what's going on, and it's impossible to determine what works or doesn't or evaluate any particular subsidy or tax on its own merit.
If you want to subsidize fertility, then make people go do the local welfare office and collect their check just like any other kind of welfare.
And measuring poverty by income is exceptionally misleading. The real measure of socioeconomic status is your spending and assets, not your income. A person who has no assets and spends 15k a year is living in poverty even if they are earning 70k a year.
And unfortunately I just opened the consumption-tax-not-income-tax can of worms.