Melatonin is a natural chemical that the body makes to let you know it's time to sleep. When you feel drowsy and groggy and start yawning, that's melatonin at work. Some people don't make enough melatonin to be able to fall asleep on their own. People who have insomnia are melatonin-deficient. I've had insomnia since birth. Many people who have Autism and other neurological disorders have insomnia and are melatonin deficient.
While I have a prescription to help me sleep, I do occasionally take melatonin. My middle daughter, who is 9 years old, has been taking melatonin for three years now because of sleep-wake cycle disturbances. With direction from her neurologist and her pediatrician, we tweak her dosage as necessary.
You can find it in liquid form or tablet form at CVS or other pharmacies. A typical dosage for a child can be anywhere from .5 mg to 3 mg once before bed time HOWEVER you MUST be sure to do this under direction from a doctor.
Before you try melatonin you really need to make sure that you're minimizing all other forms of stimulation. No TV 45 minutes before bed. Pulled shades. Relaxing activities before bed time. A warm bath or shower. Minimal books and toys and conversation during the actual bedtime routine. Absolutely no TV in the child's bed room. No late snacks. No drinks after supper time.
The reason for the major restrictions on the TV are twofold. TV stimulates the brain and that's just not necessary. Also, the light from the TV mimics daylight and it artificially extends the day in the child's brain to boot, triggering the wrong hormones and making the brain and body think that it's the middle of the day.
Wake her earlier in the morning. Open her curtains to let the light in so that her body gets used to natural light waking her. She needs to have her body retaught natural day/night rhythms.
Set up a written chart for a routine. If she can't read, use clipart photos. Include chores and expectations. Include things you're working on with her. Give her a star sticker when she succeeds with a particular task on a particular day, and when she earns a certain number of stars have a reward for her.
Once you have a new routine established, see how she does. If she still struggles then you would start with the smallest possible dose of melatonin to see if it works.