Most of the comments have been quite accurate.  This is a review study that points out that melatonin has some reproductive effects.  If you look into these, you'll find that melatonin can help normalize ovarian function, but it can also be used to "misdirect" seasonal fertility.
Overall, though, there has been nothing to suggest that melatonin is not safe in children, although I do not recommend routine use and suggest starting at .5 mg.  Even more importantly, I would look to avoidance of blue light after sunset or the use of blue-blocker glasses.   If your wild child drops like a rock after 830 while camping, you've got a hint there.
And what would the alternative be??  Pharmaceuticals like Ambien that have KNOWN problems and do not normally work as well as melatonin?  Sounds like a bad idea to me.
And what about using in kids who experience seizures or migraines?  Melatonin has some solid research behind it.  Should we avoid it here as well and use anti-seizure drugs?