It's not clear what you're giving -- 5mg or five tablets. Only one tablet a day for anyone, and 5mg is too much even for an adult. I'd consult a professional anytime I gave anything to children, but doctors aren't likely to know much about melatonin as it's mostly given by natural practitioners. But even for an adult, 1mg is a good starting point. Remember, melatonin is only for somebody with a malfunctioning body clock, and people have different body clocks. I wouldn't mess with hormones casually with a child. Have you tried homeopathy? It's very safe, since there are no active ingredients in it.
http://www.nbcconnecticut.com/investigations/Doctors-Warn-Parents-About-Melatonin-for-Kids-167339615.html