It seems that they're inevitably won by some kid whose parents just happen to be scientists, or who otherwise have a scientist in the family. And what they get that's valuable isn't just the access to the lab, it's the suggestion of a good project that's simple enough to do but which genuinely advances the field of science.
For everyone else, the science fair looks nothing like real science. They wind up spending a month playing music to a statistically insignificant number of plants, getting an inconclusive result which they will nonetheless try to sell as conclusive, and walk away thinking "Well, that was stupid".
I'm all for involving children in the process of doing real science (as opposed to the "experiments" they usually do, which are just demonstrations). But formulating and testing a proper scientific hypothesis that will genuinely advance the field is way beyond the capability of almost every child, and indeed most beginning PhD students, because they don't know where the limits of human knowledge are just yet.