Boing Boing points out a fact I've heard in several places before, but fortunately this time provided links. (Not that I think it will have any effect on the success of arguments about the topic. This is one of those urban myths that's just way too engrained.) Sugar does not cause kids to become hyper. There's no such thing as a sugar high. They even provided this helpful gif:
Unfortunately I couldn't get the link to the original article to work. Still, the point remains (as Xeni Jardin quotes in the Boing Boing post):
"A review of 12 separate research studies found that there was no evidence that eating sugar makes kids hyper."