This is a great question!
As I see you are new and you have explained within the comments above that you haven't studied biology much I will keep the explanation short and sweet.
Young children have fatigue resistant muscles and they also recover faster from high intensity workouts when compared to adults. Children have also been found to use more aerobic respiration when compared to adults during high intensity workouts. This means the child will feel less fatigue and so can continue exercising for longer. As the muscles also recover faster, the child is able to get up and start exercising again much faster than the adult as they will still be recovering.
Aerobic fitness at the muscle level decreases however once the child reaches adulthood and this explains why children have more energy and can play/exercise for much longer. They experience less fatigue and recover faster.
I hope this answers your question.