The younger a child is the less likely they are to drink to thirst, says pediatric sports medicine doctor, Jim MacDonald of Nationwide Children's Hospital. Urine color gives parents a good idea of their hydration status.
Dehydration can begin when an athlete loses as little as 1 percent of body weight. In a 70-pound child, that is less than 1 pound of weight lost through sweat. As little as a 2% decrease in body weight from fluid loss (e.g. 1.2 lb for a 60-lb athlete, 2.4 pounds for a 120-pound athlete) can lead to a significant decrease in muscular strength and stamina.