Sadly no sprogs of my own, but my ex had a couple of sons and family in Oz so had taken them back and forth a few times when they were very young.
Her experienced view was that the best strategy for transporting anybody under about 3 by air was to get a helpful friend or relative to stay up with them playing constantly for about 12 hours before the flight. This leaves said child absolutely exhausted so they should sleep right through the flight.
Something to suck is also a jolly good idea, as the action of sucking tends to clear the ears - avoiding a lot of discomfort for all involved! Dummy for very-little 'uns, suitable sweets for bigger 'uns.
G