This was me.
Well, not this flight, but a shorter one, thank god.
Ds age 6 went on a plane for the first time, and loved every minute. Calm, looked out the window, flipped through the in-flight magazine, didn't mind any of the noise etc as he had his teddy. Perfect child as you'd hope.
Way home, again fine except for when granny started winding him up by promising him a 'surprise' which always worried him. It was around this time we started to suspect he might have autism. But apart from a few sniffles in the queue for passport control, nothing any other passenger would have noticed. And the paed assessors referred him for autism assessment but the referral was rejected, as he was chatty and acing school.
So last year we were looking forward to holiday with 8yo. So was he. All fine, until he and sibling had a fight over buying a magazine in the terminal, the only coffee place took forever, and we got on the plane with relief.
At which point ds was upset that his magazine did not live up to expectations and there was a bit of kerfuffle, but then he sat by the window quietly sulking.
And then somehow he didn't notice the plane take off, was furious, and began screaming.
And screaming. As soon as the seat belt sign went off we got him onto my lap but it didn't help beyond muffling the sound for others slightly. A stewardess came to ask me to keep him quiet as other passengers had complained. Though once I'd snapped "I'm trying!" and could see the whole family were trying to distract him, she was very kind and said she would tell the other passengers they would have to put up with it. Tried to take him to the loo but he didn't want and I didn't want him flailing and hurting anyone.
He was still going once we were off the plane. Eventually DP dragged him and brother into the loos, and five minutes later they all came out and ds said excitedly "We're in (country) now!!"
He was a perfect child for the whole holiday, unfazed by all sorts of things.
Obviously we were terrified about the flight home, but ds was in total denial that he'd had any issue on the way over and was still convinced he loved planes!
So we told all the kids they couldn't buy anything as all comics wouldn't be in English, ensured he went to the toilet, and then went for shameless bribery with chocolate cake and lots of spare chocolate bars just in case. And talked the chocolate-covered child through the plane moving, takeoff, etc.
Result - happy child and other passengers and the crew who had clearly been warned. The couple who tutted about a spoilt child eating so much cake can fuck off.
It was that incident that convinced the paed to re-refer him and he did, two years later, get a diagnosis of autism, but at the time everyone insisted he had no special needs at all.