WASHINGTON (Reuters) - The Trump administration is directing immigration agents to track down hundreds of thousands of migrant children who entered the United States without their parents, expanding the president's mass deportation effort, according to an internal memo reviewed by Reuters. The Immigration and Customs Enforcement memo outlines an unprecedented push to target migrant children who crossed the border illegally as unaccompanied minors. It lays out four phases of implementation, beginning with a planning phase on January 27, though it did not provide a start date for enforcement operations. More than 600,000 immigrant children have crossed the U.S.-Mexico border without...