If this drug is not yet approve, then this is harmful for children much better if this drug is undergo first in a clinical trial and much better if it gets a positive output and medical researchers should also study and reduce its side effects.
A drug can always be prescribed "off label" when the benefits outweigh the risk.
In order for a drug to be licensed, it has to be tested on people in different age groups- generally 2-18 years and 18 and over.
It is a lengthy and very expensive process to run clinical trials and pursue licensure for an age group.
Since there are only about 50,000 diagnosed Narcoleptics, and most of us are diagnosed over the age of 18, the number of Narcoleptics under 18 is small.
So, it would not be cost-effective to pursue licensure for such a small patient population. That is the reason for the "Orphan Drug" program through which Xyrem was licensed in the first place. Treatments for "Rare Diseases" are not cost effective to research, so the government started a program to do just that.
When a drug is not licensed for under 18, it does not mean that it is not safe or effective and cannot be used. It simply has not been studied in order to gain licensure.
There are medical journal articles on Pediatric use of Xyrem.