Not my trick, but one that happened just this morning.
DH, DD (3 yrs), and I got on an elevator at our hotel. As we were trying to keep DD from pressing the buttons to other floors, an old gentleman got her attention. He had her watch as he stuffed a piece of red tissue paper into his closed fist, tamped it down so it disappeared, then had her hold her hand out under his fist. When he opened his hand, two tiny paper hearts dropped into her hand and no tissue paper was to be seen!
I think DH and I were more impressed by the trick but she was really happy just to have two new "treasures.". A very sweet way to quickly entertain a child.