I asked if it was safe and acceptable to give my 4 yr old melatonin, and she laughed heartily.
In my face!
She remembers when parents used codeine cough syrup.
Aside from melatonin there are lots of other herbs that promote sleep, like Valerian and mint.
You know Sleepy Time Tea? Its the one with the snoozing bear. It comes in several flavors now. Now they make one specifically for children. If you read the ingredients it gives an idea of what can be used on kids - hippy stuff like dandelion root.
Experiment Time...