Far-right activists are intercepting migrant children and collecting information about their families, based on an unfounded conspiracy theory that they are falling prey to sex-trafficking rings. SASABE, Ariz. — The 15 migrant children, weary and hungry, stumbled toward a gap in the rust-colored border wall that soars between Mexico and Arizona, nearing the end of their two-week trek north. Unexpectedly, a man in a cap emblazoned with a blackened American flag — traditionally, a message that “no quarter” will be given to the enemy — approached them and coaxed them to his campsite. Soon, the girls and boys, who were...