The theory is that children is less able to control their blood sugar levels then adults and that their activity levels and sleepieness is related to their blood sugar levels. That would mean that giving kids sugar before bedtime would make it harder to put them to bed. As with most theories there is some truth to it but these effects are only seen in extreme cases of blood sugar spikes and falls. This is of course easier with children as they are smaller but regular candy, and especially not juice, will not be able to produce this result.