The experiment has been done:
A) They took some kids to a party, let the parents see tables full of cake but secretly fed the kids raw tofu beans (or something like that). After dinner they made made the kids jump around to loud music for half an hour. On the way home all the parents swore the kids were hyperactive and it was all down to the sugar.
B) The took some 'problem' kids to a party and showed the parents tables full of raw tofu beans. When the parents left they fed the kids to bursting with chocolate cake, soda, anything with lots of sugar. After that they sat the kids down quietly and read them a bedtime story. The kids were falling asleep in their parent's cars on the way home. The parents put it all down to the tofu and swore to never feed their kids on sugar ever again.
Conclusion: The "sugar" thing is 100% confirmation bias by the parents.
There's a TV program on it somewhere - it's called "The Truth About Food" or something like that (it was one of a series made by the BBC).