The average person has approximately 1,400 - 2,000 calories worth of carbohydrate stored in their body and 50,000 - 80,000 calories stored as fat. (metabolicefficiency.org)
So rather than tapping into the body's own carbohydrate stores during exercise, children tend to rely more upon carbohydrate sources from food. (bengreenfieldfitness.com)
This fatigue is probably due to low carbohydrate stores which can affect performance. (sportstarsmag.com)