The American College of Sports Medicine estimates that up to 2/3 of child athletes may be dehydrated during exercise. Much like an automobile radiator, children's bodies cool mostly through "dry" conduction and convection (not evaporation): the body sends warm blood to dilated surface vessels to release the heat to the skin. A dehydrated child does not have sufficient blood for cooling in the skin and the exercising muscles.
PositiveTip: The easiest way to determine hydration is the color of the urine–it should be the color of pale lemonade.