The child tax credit, begun in 1997 as a tax cut, has become an anti-poverty program. But more than a third of children don’t receive it because their parents earn too little. The 2017 tax bill, President Trump’s main domestic achievement, doubled the maximum credit in the two-decade-old program and extended it to families earning as much as $400,000 a year (up from $110,000). The credit now costs the federal government $127 billion a year — far more than better-known programs like the earned-income tax credit ($65 billion) and food stamps ($60 billion).