Some top aides to President Donald Trump sought for months for a way to give states the power to block undocumented immigrant children from enrolling in public schools — all part of the administration’s efforts to stem illegal crossings at the southern U.S. border. Trump senior adviser Stephen Miller had been a driving force behind the effort as early as 2017, pressing cabinet officials and members of the White House Domestic Policy Council repeatedly to devise a way to limit enrollment, according to several people familiar with the matter.