Several studies now suggest melatonin can be of benefit and it is being used in some pediatric clinical settings. In this study, “only” sleep was improved of the metrics tested, but improved sleep is a benefit to children with these disorders and the parents too! 
Effect of melatonin on sleep, behavior, and cognition in ADHD and chronic sleep-onset insomnia.
J Am Acad Child Adolesc Psychiatry. 2007 Feb;46(2):233-41.
Abstract
OBJECTIVE:
To investigate the effect of melatonin treatment on sleep, behavior, cognition, and quality of life in children with attention-deficit/hyperactivity disorder (ADHD) and chronic sleep onset insomnia.
METHOD:
A total of 105 medication-free children, ages 6 to 12 years, with rigorously diagnosed ADHD and chronic sleep onset insomnia participated in a randomized, double-blind, placebo-controlled trial using 3 or 6 mg melatonin (depending on body weight), or placebo for 4 weeks. Primary outcome parameters were actigraphy-derived sleep onset, total time asleep, and salivary dim light melatonin onset.
RESULTS:
Sleep onset advanced by 26.9 +/- 47.8 minutes with melatonin and delayed by 10.5 +/- 37.4 minutes with placebo (p < .0001). There was an advance in dim light melatonin onset of 44.4 +/- 67.9 minutes in melatonin and a delay of 12.8 +/- 60.0 minutes in placebo (p < .0001). Total time asleep increased with melatonin (19.8 +/- 61.9 minutes) as compared to placebo (-13.6 +/- 50.6 minutes; p = .01). There was no significant effect on behavior, cognition, and quality of life, and significant adverse events did not occur.
CONCLUSION:
Melatonin advanced circadian rhythms of sleep-wake and endogenous melatonin and enhanced total time asleep in children with ADHD and chronic sleep onset insomnia; however, no effect was found on problem behavior, cognitive performance, or quality of life.
http://www.ncbi.nlm.nih.gov/pubmed/17242627
A recent review paper for medical professionals and interested parents to look up:
Current role of melatonin in pediatric neurology: Clinical recommendations.
Eur J Paediatr Neurol. 2014 Dec 17. pii: S1090-3798(14)00208-6. doi: 10.1016/j.ejpn.2014.12.007.
BACKGROUND/PURPOSE:
Melatonin, an indoleamine secreted by the pineal gland, plays a key role in regulating circadian rhythm. It has chronobiotic, antioxidant, anti-inflammatory and free radical scavenging properties.
METHODS:
A conference in Rome in 2014 aimed to establish consensus on the roles of melatonin in children and on treatment guidelines.
RESULTS AND CONCLUSION:
The best evidence for efficacy is in sleep onset insomnia and delayed sleep phase syndrome. It is most effective when administered 3-5 h before physiological dim light melatonin onset. There is no evidence that extended-release melatonin confers advantage over immediate release. Many children with developmental disorders, such as autism spectrum disorder, attention-deficit/hyperactivity disorder and intellectual disability have sleep disturbance and can benefit from melatonin treatment. Melatonin decreases sleep onset latency and increases total sleep time but does not decrease night awakenings. Decreased CYP 1A2 activity, genetically determined or from concomitant medication, can slow metabolism, with loss of variation in melatonin level and loss of effect. Decreasing the dose can remedy this. Animal work and limited human data suggest that melatonin does not exacerbate seizures and might decrease them. Melatonin has been used successfully in treating headache. Animal work has confirmed a neuroprotective effect of melatonin, suggesting a role in minimising neuronal damage from birth asphyxia; results from human studies are awaited. Melatonin can also be of value in the performance of sleep EEGs and as sedation for brainstem auditory evoked potential assessments. No serious adverse effects of melatonin in humans have been identified.
