Melatonin is a hormone.  It's not even safe for adults to take nightly, much less kids.  It's meant for occasional use to offset jetlag and sleep-work disorder, not everyday insomnia.

So sick of this "natural = safe" mindset.  Supplements are not regulated by the FDA like food or drugs, thus they may contain literally anything (and often do).  Some sleep aids have been found to contain benzodiazepines or other illicit drugs!  Many supplements don't even contain any of the active ingredient, so whatever benefits you're feeling are placebo effect.  This is much more widespread than most people imagine.  

If you feel good about these mystery pills to your kids, go ahead.  But parents should be held accountable should something go horribly wrong.