WASHINGTON — Ivanka and Jared left the White House behind for a life in Miami. Eric is running the family business. Tiffany got married. Barron is a budding strategist at NYU. Of all the Trump children, no one has stuck closer to their father’s side than the president-elect’s eldest son. Donald Trump Jr., 46, has found political power and personal fortune in stoking the flame of the Make America Great Again movement his father started.