We use this for DS1 (who is 5 and has ASD) occasionally.
We asked our GP about it, but he said he couldn't prescribe it as it was licensed for adults or something like that.
At our next paeditrician meeting we asked her about it, she prescribed us a months worth with no problem, only asked that we firstly gave it to him for a week straight, to ensure it worked for him and kept a sleep diary.
We now use it on nights that he is really struggling to sleep (Sunday nights for school the next day, when the clocks go back/forward) and is climbing the walls, so to speak.
We give it to him with his bedtime milk and it works better the longer it is kept in the mouth (which is great as DS1 has a habit of filling his mouth like a hamster and keeping it like that for as long as possible lately), it normally takes DS1 around 30 mins to fall asleep from finishing his milk.
DS1 has a sixth sense for identifying foreign substances in his drink/food and it is usually a nightmare to get any medication into him, regardless of flavours etc. However he doesn't seem to object to the melatonin at all, that could be because of the taste, although I really feel it is because he can't detect it in the soya milk at all.
It doesn't necessarily keep him asleep, but more helps him 'switch off' and fall asleep naturally. It also doesn't keep him asleep for any longer than normal, this means that if he falls asleep say 1 hour earlier than normal, he will be awake 1 hour earlier than normal.
I was orginally a little apprehensive about using it, but it is now a must have for emergencies in the cupboard and I will be asking for a repeat prescription, next time we see our paeditrician.
Hope this helps a bit.