Here's some info from http://www.keepkidshealthy.com/medicine_cabinet/melatonin...
Although it's available without a prescription in the USA (where it's sold as a "dietary" substitute), melatonin is not available at all in the UK or Australia.
Sleep disturbance is common in childhood, and is particularly problematic in children with neurological problems such as learning difficulties, cerebral palsy, autism spectrum disorders and epilepsy, as well as children with visual impairment (partial or complete). This is probably because these children have difficulty recognising and attuning their sleep cycle to the outside world's sleep-wake cycle.
Little is known about how useful or safe melatonin really is as a treatment for childhood sleep disturbance. A number of reports describing the use of melatonin in small numbers of children, have suggested that it is useful and probably safe, but no studies have been published where the drug has been used on larger numbers of children. Additionally, no randomised controlled studies testing the safety and effectiveness of melatonin have been performed. These types of studies are necessary to properly establish the effectiveness and safety of the drug in treating paediatric sleep problems. Evidence from adult studies suggests that melatonin generally seems very safe, but potentially it can promote depression, worsen epilepsy and delay puberty. These authors report on one of the largest groups of children with neurological problems using melatonin. (Sadly it wasn't a randomised trial).