good sedative
When our son was 18 months we took him on his first holiday but took the precaution of asking our GP what do to for flying. It was only a 2.5 hour flight but didn't want him to be a nuisance and didn't want to cause him any problems.
His reply was dose him up with medised (antihistamine/paracetamol combo) for a few days before and then give a double dose 30 mins before boarding and he should sleep the whole time.
Worked like a charm. He stayed awake long enough to watch the take off then fell asleep for the boring bit and woke up in the arrivals hall but was fairly groggy until later in the day.