At 15 and 16, they are considered as adults with regards to dosage. The only thing I would suggest is to avoid giving them melatonin.
When used regularly, it decreases the body's own melatonin production, eventually requiring to increase dosage in order to maintain the effect. On the other hand, a product like Deep Sleep from A.Vogel helps balance the production of neurotransmitters in the brain, improving sleeping patterns. With time, less of the product is needed. Here is further information about insomnia and how to treat it.