Unfortunately the last scientific study I read didn't hold this true. (It was a while back though).
In the first part, when the parents knew which their child was getting, sugar or inert placebo, the parents reported their children having higher activity when getting sugar.
The second part was a 'double blind' study. The parents didn't know what their children were being given. There wasn't any correlation between between what the children ingested and reported activity levels.
It was the parents perception.
Most times there are high sugar levels, it is also a high energy time anyway: Christmas, Easter, birthday parties...
However, this is NOT to say that there aren't children affected by sugar. It is a VERY small number.