Are you talking to knock them out as a one time deal or when they are sick or what? My son is autistic and has a horrible time getting to sleep. We give him the lowest dose of melatonin and he's out within an hour of taking it. It has been a God send to us!
To knock them out at the beginning of an illness, which is when they feel the worst and sleep the least. If I can get them to sleep for a good, solid ten hour stretch when they are sick, they invariably wake up much recovered. Its usually ultra-phlegmy coughing or a sore throat which keeps them up.