WASHINGTON — In the weeks before they were ousted last month, Homeland Security Secretary Kirstjen Nielsen and top immigration enforcement official Ronald Vitiello challenged a secret White House plan to arrest thousands of parents and children in a blitz operation against migrants in 10 major U.S. cities. According to seven current and former Department of Homeland Security officials, the administration wanted to target the crush of families that had crossed the U.S.-Mexico border after the president’s failed “zero tolerance” prosecution push in early 2018.