This quote shows something really bad, although it wasn't explicitly discussed: "But at the three fairs I’ve attended over the last several years, the unknown rarely makes an appearance. At my son's fair last year, at least a handful of students did the popular "experiment" in which the "scientist" waters plants with three different liquids—one of which is typically soda or detergent—to determine which is best for pl…
You would not see any effect, not with just 3 plants.
It's part of the problem - any effect obvious enough to show up with the small resources of a child, is also pretty obvious.
I would do different: Have kids reproduce famous known experiments, and get rid of this useless obsession with hypothesis first. Just do a "hey, that's interesting" type experiments.