A perfect trick for children, but with a different presentation, it's fun for adults, too!
Show a wooden frame having two doors. Place a card in the left door, and state that you will make the card disappear. Tilt the box slightly to the right, and the audience can hear the card sliding over to the other side. Open the left door and the card is gone. The audience cries out for you to open the right door. Show them they're right - you cheated! The card is on the right side.
Much byplay goes on with opening doors and tilting, causing the audience to scream more and more - you keep opening the wrong door. Finally you open both doors at once - the card has VANISHED! Much fun. Mystifying for all!