i've worked with kids since i was a student including summer camps, activity centres, guides and brownies etc. and i have always known that the sugar thing was rubbish for 99% of children.
kids work themselves into a 'sugar' frenzy because someone has told them they will be hyper if they have sugar... you can get them to do it even if they haven't had any [evil grin] and you can also feed them sugar secretly and they don't get hyper at all.. when you have kids on camp for a week you deliberately hype them up at times for games etc. and calm them down again and it's all suggestion, honestly.
i know a FEW children have an intolerance to certain ingredients and i'd never mess with that but i think this is usually e-numbers linked to colour rather than sugar.