So say your couple A gets $100. But that $100 had to come from the bureaucratic machine that is the government. They had to hire extra people to deal with that incentive. This $100 that you get probably costed taxpayers $200 so if you scale that up somewhere people fall below a reasonable treshold where they were pondering to have children but decided to not do it because even though they get $100, they are charged $200 extra in taxes so they are still $100 short.
Taxes always weigh heaviest on people just entering the job market for minimum wages and these are the people that have to make the children.
Even with a 1:1 relation where you get taxed $100 and somebody else receives that exact amount without intermediaries taking cuts it would still make no sense. You are quite literally destroying capitalism and the free market right there.