I have 3 kids under the age of 6 and I call a total bullshit!
There are many recent studies that try to dismiss a correlation between sugar and hyperactivity and most of them appear to be pretty flawed. One study states that the parents of the children in the study did indeed report that sugar correlated to hyper-activity, but dismissed that in favour of "clinical" reports--so in their normal environment kids become hyper after eating sugar, but when you put them in a lab they don't. Perhaps the change of environment has an impact on this? This study also compared sucrose, aspartame, and saccharin based on THE ASSUMPTION that aspartame and saccharin do not cause hyperactivity. But few if any of these studies administered the test dose on an empty stomach and monitored the biochemical response.
Other studies show that since sugar is quickly absorbed into the bloodstream, blood sugar rises quickly, which can lead to higher adrenaline levels and thus symptoms similar to those associated with hyperactivity. This fits much better with what we understand about metabolism and is not based on any assumptions or other external effects, such as change in environment.