Kid's energy vs Adult's energy

What is it that makes kids more energetic than adults? I mean their bones are more fragile, their organs are smaller, their food intake is less yet my kids always beat me every single time we work out together! I would download a new dance/workout video and by the time I'm halfway through the routine, I'm covered in sweat and panting while the girls are sweating but smiling happily as though they can feel no pain! How come? This question has been on my mind for quite some time now and then I decided to ask the all-knowing Google and voila! I got my answer;

Actually kids are more efficient in synthesizing energy from their food intakes because of their metabolism. So they are more energetic than the adults. They can synthesize energy faster than the adults{normally}. ... So they are more energetic than the adults - https://www.quora.com/Scientifically-why-do-kids-have-more-energy-than-adults

Another article said they are more energetic because they have access to much more oxygen than the average adult. The same article also said they are less stressed out and are able to breathe much more easily than adults; they release tension, toxins and stress through breathing and so they invariably have more energy. Interesting stuff eh?