Why do adults need more carbohydrates than children?

My son can eat like a grown man. They need more food than an adult because they are growing so fast that they just need to eat to keep up with their bodies. my son is very active and eats all the time and is still, at age 5, only 39.5 pounds. so let them eat but only the healthy stuff. dont want our children togrow up and get made fun of.