Why do kids need more sleep than adults?

Children need more sleep because their bodies are developing very quickly. Therefor, they need to have the chance to give their bodies a rest as often as possible.

This of it like this,

Two people are going down a road, one walking, the other running as fast as they can. The person who is going faster would need more rest afterwards, right? Well, that's the child. :)

Hope this helped!