Sorry, don't often post on here but I saw this thread and hope you don't mind me gatecrashing.
Maybe we're just lucky but we've been flying with our son (who's almost 6 years old now) since he was 6 weeks old. DH's family live in Scotland, we're based in London so flying was the quickest option. He's flown 3 -4 times a year to Scotland alone. The only time we've had a problem was when he was teething and the ear pressure got too much. We've been lucky enough to go to Austria, Lanzarote, Italy and Spain during that time too.
We time our flights to coincide with his sleep time whenever we can. We take a bag on board just for him and fill it with books, toys (like small cars, nothing noisy as it'd drive me insane!) and food and we never take everything out at once. One of us will walk him up the aisle a couple of times and before the flight we walk or run him around the airport to let him let off steam. You'll sometimes literally see one of us running the little fella up and down in the airport lounge! Seems to get it out of his system!
When he was almost four years old we inherited a little cash and used it on a once in a lifetime holiday (before he started school and we were limited to school holiday times). We went to Hawaii (a 13 hour flight), Australia and New Zealand for a total of 6 weeks. He had to fly from London to Los Angeles, next day to Hawaii. A week later off to Australia (via New Zealand), then after five days, back to New Zealand for the remainder of the holiday. Then the 27 hours home (stop off for a couple of hours in Hong Kong airport). We spent almost as much time getting the right timing on flights as we did on the holiday itself - again trying to get local time evening flights when we knew he'd sleep (though the Los Angeles flight saw him awake for 20 hours!)
He discovered the kids channel on the TV (hooray) so we had an hour or two's peace every so often.
The people behind us stopped me as we were getting off the 27 hour flight home and my heart sank. "Your boy was better behaved than some of the adults" said the woman. I felt so proud!
The point is, as he's never been given anything and we've worked out what works for him because he's been his usual self not sedated in any way. He now sees plane travel just like any other form of transport. He understands how he should behave because we've taught him and he's been awake enough to take it on board. If you give sedatives to your kids, at what stage do you test the water to see if they can behave? At what stage do they learn? It's like saying I won't take my kids out for a meal until they're older. They'll still won't understand the 'rules' about eating out because they've never experienced it. Sometimes you have to bite the bullet. You won't get it right first time but you soon learn what works for you and, more importantly, for the kids.
By the way, to add to the mix, we were in the centre of Christchurch on our last day of holiday in New Zealand when the earthquake struck. We ran our boy through the streets with glass showering on us and cracks opening in the pavement near us. All night we had aftershocks (every 40-60 minutes) and the airport had damage to ceilings, had people being flown home by the Red Cross so were 'walking wounded' and in shock and the airport had aftershocks too. We were all holding our breath as the plane took off in case another quake struck as we were taxi-ing. And still our boy didn't howl about the plane, run up and down or kick the back of the chair in front. He knows the rules, they make him feel safe so we stuck to them even after all that and it helped him settle. Make the rules and stick to them, no concessions, no changes and the child knows what is expected and, more importantly, what to expect.