Safe way to extract hydrogen from water through electrolysis
Some sort of advice, since you are concerned with safety and demonstrating the experiment to children. You should generally not perform an experiment in a badly ventilated space. Please be aware, that the generated hydrogen gas is highly flammable and with the addition of the right amount of oxygen, it can make very loud booms: How explosive is hydrogen gas?

Here are two questions, that are very closely related and already contain the answer to your question:


Can I perform electrolysis without electrolyte? (with distilled water)
How can I produce hydrogen gas efficiently and cheaply?


In general any kind of salt as an electrolyte will do.

In low concentrations about 1-2 mol/L they are usually quite harmless. I personally would prefer diluted sulfuric acid. Any addition of group 2 salts may cause passivation of the electrodes by trapping carbon dioxide from the air. 

Diluted sodium hydroxide or potassium hydroxide are also safe to handle.

Probably the safest is sodium sulfate, but it might not be the best performer.

Please wear safety goggles at all times during the experiment, best case: The children wear them, too.