This project won't require much, it's pretty easy.
We'll need something that can propel a lot of air, like a Blowdryer
or a leaf blower
. These don't have to be anything special, although you do get a little bit more power out of a gas-powered leaf blower.
Some open space if you're going to use a powerful blower, (I suppose you could use an electric leaf blower inside, though).
Something to levitate:
Table-tennis balls work well with the hair dryer, but other small, light things can work, too.
Using a leaf blower opens a lot more possibilities, like beach balls (these are the absolute best if you can find one ((or two!))), or two liter bottles, etc. (Anything without too many corners, and isn't too heavy. Think about how much a small apple weighs, and you have an idea of what we're looking for.)
Some ear protection for the youngsters is a good idea.