I was sat next to a kid who was clearly bored out of his skull. His parents weren't particularly interested, so he and I swapped seats so he could look out of the window. Once I had told him how to look for interesting stuff, he turned into a very interested, and interesting, person himself. I fly a lot, and love looking out the window. There's an entire planet passing by, and not looking at it is a missed opportunity.
Something I've done with friends' kids is give them a disposable camera, and ask them to take photos of anything interesting they see en route, including a photo of the airport they arrive at. They have to decide what's interesting, and also pace themselves.
Kids are great. They never, ever want to stop learning. They can do a heck of a lot in an airplane without annoying anyone.
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