Trump’s proposal for a new tax break for parents, as I wrote here yesterday, makes sense as a way of reducing what Senator Mike Lee calls the “parent penalty” in federal law: the way the federal government overtaxes parents, and especially parents of large families. But I also noted that it would involve a big revenue hit.

Ryan Ellis, whom I cited in yesterday’s post, did some back-of-the-envelope calculations:

According to the IRS SOI data, there were 83 million dependent exemptions for children at home. Now, this new deduction is only for those under age 13, so let’s cut that number by a third to 56 million new deduction items. Let’s bring that down to 50 million to account for the means test and to be conservative about this.
According to the source I read yesterday, the average price of child care is $12,000 per child.
$12,000 x 50 million is $600 billion. Per year. Multiply that by a 25% marginal tax rate, and you have an annual tax reduction of $150 billion.
Over ten years, I would guesstimate a score of $1.8 trillion over 10.

When Senators Rubio and Lee proposed expanding the child tax credit, some economic conservatives objected that it would reduce revenue too much–and said that the space in the budget could better be spent on pro-growth tax cuts or deficit reduction. And that proposal was estimated to reduce revenue by significantly less: $1.22 trillion over a decade. (That proposal would also have been worth more to a lot of middle-income families.)