Whats better 3mg or 300mcg melatonin a good dose for a 9 yr old autistic child?

Start with 300mcg. Melatonin can have different effects on different individuals, regardless of size. Adult sized patients can be dramatically affected by 3 mg and children not at all and vice versa. Start with the 300 mcg dose which is 1/10th the 3 mg dose.
Good dose below. There have been MANY different studies using Melatonin for Children with sleeping disorders, especially in children with autism. Dosages used have been in the range of 0.1 mg up to 6 mg with good results at improving their sleeping problems. Melatonin is safe and effective even in children with severe disability, those who are taking psychotropic drugs and in those who have a history of seizures.