This is neither something that I'd recommend to either your average 5th or 7th grader, nor would I recommend it to the parent of these children, but, since you seem to be one of the most ambitious people, I've ever met
I'm going to throw this out there.
Find a college lab that will give you access to their gas chromatograph, and, for once and for all, put the 'it's the water' idea to bed. I think most people are aware of the water myth, and, while it might not generate a whole lot of excitement, it would be an amazing experience.
That is, if you can talk a college lab into helping you out (or any lab, for that matter). You hear stories about people befriending lab techs and scoring liquid nitrogen for home use- this is quite a bit beyond that.
Perhaps you could trade some lab time for pizza