0.3 mg won't touch sleep disturbance, IMO. DD1 takes melatonin each night, 3mg orally (liquid), and she weighs 18kg.
I think it's worth bearing in mind, that around 1/3 of children with learning difficulties of any kind are thought to also have epilepsy. Epilepsy can first appear in late toddlerhood, which is when children are deemed to be 'abnormal' if they haven't started to sleep appropriately at night.
It is logical to me, that the very fact that a child has LD, or SEN of some sort, and develops epilepsy, at the same time melatonin is prescribed can be pure coincidence.
DD1 was diagnosed with epilepsy at 3.1. She started melatonin at 4. Had she developed her epilepsy a year later, would they have attributed it to the melatonin? Possibly.
It's the same as painkillers that cause headaches, cure headaches. Or antisickness drugs which cause sickness...well the people taking them were sick!
Melatonin (synthetic) is safe, and merely a reproduction of our natural productions. Don't buy from America mail order, though, because they often use animal melatonin, which isn't known to be safe.