It’s not that they have more energy. They just don’t conserve energy like adults do. Just like a juvenile snake will put all of its venom in one bite children use up all of their energy and then collapse in exhaustion.

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That's my experience!

We like to walk to get breakfast on weekend mornings, it's about 8 blocks away. My 3yo tears out the door at full speed and runs circles around everyone for the first two blocks. Then for the next 6 blocks she drags her feet, asking us to carry her because she's too tired. We end up either carrying her a bit or stopping every block to take a long rest break.

Makes sense, but do they not also use this energy to grow?

Juvenile snakes releasing all of their venom is a myth

Yes but not always and they also don’t need as much food to get enough energy to move their smaller bodies