Unfortunately, the secret of this trick is now well known due to the selling of "magic sand" at toy stores this year.
You often hear it said that the secret of an effect is not the most crucial thing. I would now however, be hesitant about performing this trick for a group of children.
I'm reminded of the Vernon story about the orphans and the vanishing candle trick ...
PETRIE-LEWIS! PETRIE-LEWIS! :D