I believe it's not generally considered valid to apply results from medical studies on adults to children. I'm not sure if the reverse applies.
The fact that the study was on children certainly doesn't help the validity when applied to adults, but I think you're being overly risk-averse. Melatonin's mechanism of action is pretty well understood, and it occurs in the body already. The long-term effects would have to be very bad to outweigh the advantages of a regular sleep schedule and an extra hour of wakefulness every day. That's assuming melatonin works, of course.
Full essay: http://www.gwern.net/Melatonin