Thanks for the information, Doodle. I am 10 years retired from being a nurse practitioner for children with special needs, so a bit out of date, but understand the principles.
However frustrating, it is quite right that prescribing is initiated and overseen by a specialist.
A lot of medication for under 18s is not licensed, it is so difficult to do trials. Specialist paediatricians will prescribe 'off licence' and so will GPs and nurses once the system is set up, under certain circumstances.
Melatonin is a strange substance - small quantities work well in some children, and large doses have no effect in others. It seems so variable. Also, it doesn't have a long , well documented history of being prescribed in adults, so there isn't much knowledge around its long term problems.
Adolescence is also a difficult time for prescribing anything as brain function seems so changeable.
Please do ask if you can have input from a specialist nurse, as they are usually very experienced, can make helpful practical suggestions and make a relationship with the patient and family.
Good luck!