Preface: This is horrible, but please don’t scare-monger about melatonin.
Melatonin is a hormone the body produces naturally to regulate the sleep cycle, and it tends to stabilize sleep rather than induce it (based on personal experience).
Also, a former professor of mine, before deciding to take it himself, gave rats massive doses of the stuff with little-to-no ill effects. It’s not a scientific study by any means, but it definitely suggest to me that it’s safe.
Epilogue: Don’t drug kids, even with substances safe in massive doses.