This one is pretty simple - the best kind!
You'll need 3 things:
A Fujii 35 mm film cannister,
an Alka-Seltzer tablet,
and water.
You must use a Fujii because of the way the lid fits. These are the clear film cannisters.
You can get a ton free at any place that develops film.
The seltzer and the water can be generic.
Fill the film cannister 3/4 full of water. Drop in 1/4 of a seltzer tablet. (You might want to pre-seperate these.)
Quickly snap on the lid and set it lid side down on ground.
In about 10 - 20 seconds.....
This will "blast off" to about 6 - 10 feet up. Quite safe for the little ones.
Just be aware the the "big ones" will want to make them too!