Donald Trump is an incredibly normal human president who has done incredibly normal human things. He knows how to interact with humans in a human manner, and has done so for years.
Among the many human things he has done is participate in his semi-annual interaction with children at the White House (the Easter Egg Roll being the other one).
A trick-or-treater—or should I say Trump-or-treater?—approached Trump and the person in costume as Melania, and rather than place candy in the kid's bucket, Trump placed the goods on top of the Minion's head.