For years, the bright green turf of Whitsett Fields Park has served as a joyous hub for Los Angeles youth soccer — particularly for thousands of immigrant families in the San Fernando Valley. On most weekends, the sprawling North Hollywood complex echoes with the shouts of hundreds of boys and girls, as vendors hawk aguas frescas, balloons and candy along the sidelines. But recently, immense grief and worry have settled over this close, Latin American community. Just last week, a well-known coach and Salvadoran national was charged with murder in the killing of 13-year-old soccer player Oscar Omar Hernandez during...