E.M.
I have flown dozens of times with one or both of my daughters as infants and toddlers. It isn't always easy but I have never given them anything to make them sleep-except once when my older daughter was like 10 mos. Nine months shouldn't be too hard an age-some books, some toys, lots of snacks.....and if you can keep her awake until just before you take off, the white noise might put her right to sleep. I have never heard of using melatonin for a child so young and the one time my daughter had benedryl at around 10 mos (this was before they said it is not safe for children under 6) it was a disaster. She fell asleep right away but woke up shortly after jittery, groggy and screaming for the rest of the flight.