The U.S. Border Patrol earlier this month arrested a 12-year-old illegal immigrant smuggling 80 pounds of marijuana from Mexico into Texas, reigniting a debate over controversial comments made by a Republican congressman earlier this year about children’s involvement in the cross-border drug trade. Rep. Steve King, an Iowa Republican who has called for a crackdown on illegal immigration, had said there were more young illegal immigrants smuggling drugs than there were ones who were valedictorians of their classes in U.S. high schools. In the Texas incident earlier this month, Border Patrol agents arrested six Mexicans — two adults, two 17-year-olds,...