Only gone as far as HK.
Suggestions: take a bottle (sports style, not teated, unless thats what they are used to), and fill it with a prefered drink when past customs (we bought a pint of milk, which was OK for the 12 hrs without a fridge), take loads of snacks - they will sometimes want food at inconvienent times, take sticker books, makes sure anything you take doesn't stop working if you loose one small piece!
Lots of adults were walking round with small children - the kitchens at the back of the plane seemed to be the gathering place!
We had bulk head going out, and DS was small enough to be offered a bassinet at 20 mths (!), he was pretty dinky tho.
Coming back, we had normal seats (bought him one, even tho we didn't have to), and put armrest up, and he slept with his head on my lap.
Sachets of calop were our saviour coming home - it was the only think that stopped him screeming, so I assume he really was in pain.
Don't stress about it - everyone was lovely.
Enjoy Aus!