We will set a dollar bill on fire and it will just end up being wet.
Procedure:
Mix equal amounts of 95% alcohol or lighter fluid and water in a container.
Place a dollar bill into it until it becomes wet.
Remove the wet dollar with tongs or a large set of pliers and shake off the excess liquid.
Darken the room and then light the dollar on fire. The alcohol will burn off, but the water will protect the dollar.
You could also suspend the dollar on a coat hanger. If you use vodka or gin then you do not need to dilute with water. A variation is the burning water trick below.
Take a glass and swirl a small amount of alcohol in it before a child sees it.
Then fill the glass half way with tap water when the child can see it.
Light the surface on fire immediately with a long match.
Questions:
Ask for observations such as the presence of heat, smoke, flame, odor, etc. See if they detected an oil layer on the surface of the water.