The creature Sophie sees kind of looks like a human, only he's four times as tall as one. She was definitely right about the Witching Hour.
He's carrying a trumpet and a suitcase.
Sophie watches as the giant peeks into the window of a family across the street, where two children live. The giant takes a glass jar from the suitcase, puts its contents into the trumpet, and silently blows it into the room.
Then the giant sees Sophie. Oops. Sophie runs back to her bed, but we all know it's too late by now.