I have two children that have sleep issues. One has problems with insomnia along with a lot of other stuff, and the other takes meds for ADHD that end up getting him hyper at night.
Both take 2 mg of melatonin. It works ok to get them to go to sleep, but not always to stay asleep. The younger one is 7 and his pediatrician wants him to try 3 mg. He's very small for his age so I've been dragging my feet. The older one is 10 and has had insomnia since 18 months. He takes tranquilizers which help keep him asleep, but we need the melatonin to get him relaxed enough. He is big for his age, so we will be moving on to the tylenol/benadryl combo. It ends up equivalent to tylenol pm and not as addictive as other things.
The trick with any sleep assistance is to make sure you try it for a week or two before making any decision. Also, it may take a combination of things to make something work better.
Each person's body is different and will therefore react differently for everyone. A good suggestion is to talk to a pharmacist about OTC assistance that won't cause dependence or a need for a physician's visit.