Popular Mechanics bought eight science kits for kids, reviewed them, and found instructions on the 'net for replicating the same experiments with materials many people have on hand.
http://www.popularmechanics.com/science/research/4323417.html -via Geek Like Me
...homemade experiments can be just as complex and educational (while costing up to $100 less), so we found alternatives to each of the boxed kits that teach similar lessons just as well. Bottom line: Whether preassembled or drawn from kitchen cupboards, science kits can be educational and fun.
http://www.popularmechanics.com/science/research/4323417.html -via Geek Like Me
If one is capable of explaining the electrolysis of water to Hydrogen, then use the homemade experiment. Otherwise, you would be smart to buy the kit.