As they say, poison is in the dose. Try pouring all that corn syrup and artificial flavor down your kid's throat.
It's the quantity of liquid more than anything. This can happen with drinking plain water. It dilutes the electrolytes (salts) in the blood, and leads to mental confusion and possible death if untreated.
According to this article, an adult can safely drink up to a liter (about one quart) per hour.