A protester in Hartford, Conn., in January. (John Moore/Getty Images)

INFANTS, TODDLERS, tweens, teens — Trump administration officials are less interested in the age of an unauthorized child migrant than they are in removing the child from his or her parents as a means of deterring illegal border-crossers. That plan, first floated by White House Chief of Staff John F. Kelly last year when he was homeland security secretary, was widely regarded as so callous and such a radical departure from historical practice that it was unthinkable for any U.S. government.

If only.

In fact, not only has the idea of systematically separating undocumented children and parents gained currency among top officials determined to turn the tide on illegal entry, it’s already happening with increasing frequency. The Department of Homeland Security insists it has not adopted the practice as a matter of official policy — despite White House pressure to do so — but administration officials acknowledge that hundreds of children, including scores younger than 4, have been taken from their parents in the past few months.

By now it’s clear that there are few red lines President Trump is unwilling to cross in his crusade to rid the United States of undocumented immigrants. For Mr. Trump, having washed his hands of the “dreamers” — young migrants, most in their 20s, raised and educated in the United States after being brought here as children — it’s hardly a moral leap to inflict lasting psychological damage on younger children by taking them from their parents if it will further his goal of combating illegal immigration.

As reported by The Post’s Maria Sacchetti, top immigration and border officials have recommended that all parents who enter the country illegally with their children be detained and prosecuted, meaning the automatic separation of minors, who cannot legally be held in jails or detention centers designed for adults. Until recently, that was extremely uncommon; most parents who crossed the border with children would be released pending an immigration court hearing, or, in some cases, detained together in a facility designed for families. Prosecuting parents for illegal entry, a misdemeanor under federal law, has been exceedingly rare — specifically because of the harm it would cause blameless children.

In addition, many of the parents who would be prosecuted are eligible under U.S. law to seek and be granted asylum. That’s hardly a stretch for migrants from El Salvador and Honduras, beset by drug cartels, gang violence, domestic abuse and some of the world’s highest homicide rates. In the last three months of 2017, more than two-thirds of the 30,000 asylum seekers crossed into the country illegally — and it is far-fetched to exempt from prosecution only those who announce themselves as asylum seekers at legal ports of entry, as Homeland Security officials propose. Are desperate, impoverished people fleeing violence to be penalized because they enter the United States in the wrong place?

The United States has a legitimate interest in deterring illegal border-crossing. It is within its rights to detain and deport individuals and families who fail to make a persuasive case for asylum. But to splinter families and traumatize children in the name of frightening away migrants, many of whom may have a legitimate asylum claim, is not just heartless. It is beyond the pale for a civilized country.