Baking soda and vinegar eruptions, glue and borax plastic, and glowing petroleum jelly are all experiments appropriate to elementary school children. The experiments also benefit from having easily obtainable and fairly inexpensive materials.
Adding vinegar to baking soda creates vigorous fizzing and eruptions. Using a narrow-mouth bottle to contain the reaction and adding a bit of liquid soap results in a reaction that erupts from the mouth of the bottle like a volcano. The mixture of baking soda and vinegar creates carbonic acid that almost instantly breaks down into water and carbon dioxide. It is the escaping carbon dioxide gas that is responsible for the eruption.
Dissolving borax laundry powder in water and combining this mixture with white glue results in a soft, moldable plastic, similar to silly putty. Glue itself is a polymer, a composition of long chains of molecules. Borax becomes part of the chains and links them together to form a three dimensional structure that makes the new substance behave more like a solid than a liquid. Clear gel-like glue also works for this experiment but gives a slightly different consistency.
Ultraviolet light is invisible to humans but is responsible for sunburns. Black lights also emit ultraviolet light. Petroleum jelly contains compounds called phosphors which absorb ultraviolet light and re-emit it in the visible spectrum. Because of this, petroleum jelly gives off a greenish glow upon exposure to a black light.