I've flown with all my kids (4 of them when aged 18 months to 9 years most recently) so have some idea of techniques that work.
My second eldest is fearful of just about anything new and on our last trip (BNE to MNE in Oz - a touch over 2 hours) was absolutely beside herself with fear. I suspect she was picking up on my wife's anxiety - she is a quite anxious pax.
The tactic I adopted was to explain as much as I could (from my own feeble knowledge and tidbits I have picked up from pilots) about what was going on during the flight, explaining all the noises [that I knew anyway!], how high we were, how cold it was outside, how fast we were going etc and pointing out landmarks.
She was so intrigued by the whole process that by the time we were on the return flight she was explaining what she knew to her mother - "We are just taxiing now mum ...."
None of this helped my wife though
