Why a U.S. Childfree Tax Could Be Politically and Socially Unworkable
I don't think the U.S. could realistically impose a tax specifically targeting childfree adults, even if there were political support for pronatalist policies.
Parents also have to think about their own children's safety. Before supporting policies designed to pressure people into having children, they should consider the risks their children could face, including the persistent problem of school shootings in the United States. Asking people to become parents while failing to adequately address those risks raises serious ethical questions.
The U.S. also has an unusually large, armed civilian population and a deeply entrenched culture of individual rights. A policy perceived as coercing people into having children could generate enormous political resistance and potentially destabilize society. I wouldn't assume that childfree people are inherently violent or that a civil war would be inevitable, but policymakers should take the possibility of severe backlash seriously.
If the goal is higher fertility, incentives such as affordable housing, childcare, parental leave, and direct family benefits seem much more practical than punishing people who don't have children.
We don't want a bloodbath. A few people allegedly freeriding the system is much better than large groups of people purging a lot of old folks and children who can't defend themselves.