The U.S. refugee agency has discharged hundreds of children in recent weeks, with the number of minors in its care dwindling from more than 3,500 in early March to the current 2,100 in-custody population. But the agency has also been receiving very few children from border officials, who are now expelling most unauthorized migrants, including minors who arrive at the border alone, by invoking expansive turn-back powers under a public health order they say is designed to curb the spread of the coronavirus.