Is Sugar a stimulatant or a weak sedative?
The idea that 'dyes and preservatives' cause hyperactivity dates back to this '76 paper by one Dr. Feingold. Despite being disproved neatly in 1978, it's still around.

As far as sugar goes, I recommend this review paper covers both the arguments for and against sugar causing hyperactivity, as well as 12 double-blind placebo controlled studies showing sugar has no effect on children's actual behavior.