I recall DS1, 11 months, howling pretty much non-stop from Tokyo to London. We (DH and I) had spent a very committed 8 hours keeping him 'entertained' from Sydney to Tokyo, then made the error of a night's stop over, leaving Tokyo at noon, him 'refreshed' after a night's sleep. SO DH and I spent the 11 hours tag team walking him up and down the aisle. Luckily, the plane was full of Japanese who looked pityingly at me rather than tutting and huffing but still. It was a nightmare. Especially when the 9 month old baby of the adjoining couple pretty much slept the whole way, only waking to eat, gurgle and coo....
SO I was absolutely dreading flying DS1 (then 4) and DS2 (2) from Sydney to London 3 years later. We planned the flights as well as possible; 4 hours to Brunei, a 4 hour run around at the airport, a 7 hours stretch to Dubai (where they sort of slept, me sitting on the floor in front of the 3 seater so the DSs could stretch out!), an hour in Dubai airport (at 1am local time, Dubai lit up like a Xmas tree!), then 7 hours to London where again, they slept for 4 hours, then the hard work of the Last Stretch, me bringing amusement after amusement out of a backpack, but thinking, as we overflew Romania, only 3 hours out, "Nearly there! Nearly there!"
But, of course, flying them to Sydney 2 years ago, aged 10 and 12, on a brand new Airbus 380, back-of-the-seat entertainment, was a breeze.
This too shall pass.