Magnesium has worked wonders with our Grandson. His Mom actually makes up a cocoa butter/epsom salts/water mixture (so the magnesium will continue going into his system for a long time). It's a little effort and feels kind of grainy on, but I'll send the recipe if you'd like. Our Grandson doesn't seem to mind the "feel."
Easier: try giving him "Kids Natural Calm" before he goes to bed. It's mainly magnesium and is a powder to be absorbed quickly.
Magnesium in an easily absorbable form has also been said to produce great results with ADHD. It should be given throughout the day (not all at once), just like vitamin C.
Melatonin, or any other over-the-counter preparation, should be given to children under the guidance of their pediatrician. In general, melatonin does appear to be safe for use with children, and it is sometimes advised to address insominia with children who are prescribed psychostimulant medication to treat ADHD. As you probably know, with some children onset insomnia is a symptom of their hyperactivity; for other children it might be a side effect of medication. As you probably also konow by the research you have clearly done, inadequate sleep can produce symptoms very much like those of ADHD, and it can also produce changes in mood and in cognitive functioning. So I'm just suggesting you bring the pediatrician into the planning.