I discuss melatonin's effects on sleep & its safety; I segue into the general benefits of sleep and the severely disrupted sleep of the modern Western world, the cost of melatonin use and the benefit (eg. enforcing regular bedtimes), followed by a basic cost-benefit analysis of melatonin concluding that the net profit is large enough to be worth giving it a try barring unusual conditions or very pessimistic safety estimates.
Full essay: http://www.gwern.net/Melatonin
I have two kids. If left to their own devices, they would eat the tastiest things on their plate, then stop (then complain about being hungry an hour later). They would never eat anything remotely healthy, and subsist entirely on chocolate if given the choice.
Since we have evolved to value fat and sugar as being the tastiest substances, children do have to be taught/persuaded to eat healthy food.
They also do need to be told when to go to bed. The times at which we have tried to let them set their own bed times have resulted in them trying to stay awake as long as they possibly can, until they fall asleep in the middle of whatever they were doing. They almost never voluntarily go to bed, no matter how obviously tired they are.