I was concerned when I read this article because I know so many parents who use this instead of medicating their children. I asked my MIL to review it for me since she's worked as a PhD in biochemistry in the science field for years. Here's what she had to say.......I read through the article you mentioned warning the public that a researcher, Professor David Kennaway, who has worked on/with melatonin for 40 years published in the Journal of Paediatrics and Child Health. The actual title of the article was not given, as far as I could tell—nor the date of publication.
Let me provide several quotes and comment on them, but first let me say melatonin is a natural body substance –i.e. all our bodies make it in a greater or lesser degree and it is one of our sleep regulating hormones.  Substances our bodies normally make are not federally regulated because they are a normal part of our make-up and not considered dangerous.  That said, many things in our bodies can become problematic if large does are given, but that doesn’t seem to be the issue here.
In a paper published in the Journal of Paediatrics and Child Health, Professor David Kennaway, Head of the Circadian Physiology Laboratory at the University of Adelaide's Robinson Research Institute, warns that providing melatonin supplements to children may result in serious side effects when the children are older.
This is a generalized statement, made without any evidence since he says later:
“…there have been no rigorous, long-term safety studies of the use of melatonin to treat sleep disorders in children and adolescents.”
The author does say:
“…there is extensive evidence from laboratory studies that melatonin causes changes in multiple physiological systems, including cardiovascular, immune and metabolic systems, as well as reproduction in animals.”
And the author gives one example—stating that melatonin is used:
“…for changing the seasonal patterns of sheep and goats, so they are more productive for industry.” 
What he does not go on to say is that humans and animals are designed fundamentally different sexually.  God set into the bodily rhythms of animals seasonal cycles so offspring are born when weather is warmer and food more plentiful so the young can survive—i.e. typically in spring, not fall or winter or late summer.  Human on the other do not have “rutting season” as sheep or goats, but are sexually active all year long and have the creativity to care for children whenever they are born spring, summer, fall or winter.  He also made no reference to the size of dose necessary to overcome the natural cycle and how it compares to the amounts of melatonin that is recommended for children.
Professor David Kennaway also states that:
　　　　　　　"There is also the potential for melatonin to interact with other drugs commonly prescribed for children, but it's difficult to know without clinical trials assessing its safety.
This is a pretty safe comment to make, since everything in our bodies or that we take into our bodies has the potential to react with everything else.
This young man’s comment on the article was pretty accurate:
I nearly fell asleep reading this (pun intended).
Elliot Shayle  says:
April 9, 2015 at 12:27 AM
I'm 19 and I frequently take prescribed Melatonin for the past few years. However, this article seems rather insubstantial. It's mostly just quotes from this professor, with little substantiation behind them. I fear elements of this may be taken out of context or otherwise mentioned in an incomplete fashion. I'd like it if the authors had provided greater insight into the side effects of Melatonin, and critically analyse the professor's comments with regards to other published material on the subject. This article seems like clickbait. A lot of speculation, very little substance
I think when a child can’t sleep, it might be wise to give some time and effort to looking into other factors that may be causing the sleep problem. Such as,
Is the house quiet at their bed time?  Is the TV off, or is there low level interruptive sound reaching their bedroom?   Provide white noise—like soft religious music Bob Daniels is great for this if his recordings can still be found or Patch the Pirate bedtime CD.
Are they watching TV programs that have scary things in them—not scary to an adult but maybe to a child.  
Low level fears, worries and concerns come out in all of us at night and plague our minds –is something bothering them.  Is there any possibility of abuse where they are during the day?  I would urge parents to sit with them 10 or 15 minutes at bedtime when they are getting ready to go to sleep and let them talk, see what  they want to talk about, what is on their mind.  It may take a little while to come out.
Bottom line—is the administration of melatonin a possible problem?  Answer we don’t know, but lack of sleep or poor sleep is a problem—I wonder if there is some other reason it is happening.  Has it always been a problem?  Did it develop recently? Possibly related to something else that changed in the child’s environment.  If a parent is using it long term they should stay in touch with their pediatrician about it.  
Best Mom B  [alias Mrs Dr Brown :>) ]