The goal of this exhibit would be to show how liquids evaporate quickly and boil at lower temperatures with reduced vapor pressures. This would be shown by having a liquid "disappear" when the container's pressure was reduced (and volume increased).
The structure of this exhibit would consist of a clear tank, filled about 1/5th full with some liquid (water perhaps). The top of the tank could be moved up or down by some sort of crank or mechanism that the child would control. As the top moved up, a partial vacuum would be created (the vapor pressure would decrease) and the water would evaporate or if conditions were right, it may boil. Hopefully getting all the liquid to vaporize would not be too difficult to do or reverse, so the child could repeatedly do it.