I did a 4 hr flight with a 3mnth, a 2yo and a 6yo, and it actually wasn't as bad as I had feared.
They did sleep some of the time, and I did what JackBauer has planned with the bits and bobs and it worked brilliantly.
I wrapped everything up separately and put it in a small duffle bag each(not the baby obviously!). I told them that when they had been good for a while, they could pick another present out of their bags.
They were really excited to be carrying these little bags of secret goodies, they had treat foods in there too that I would not usually let them have(lollies, wotsits, those manky filled cracker things, cheese strings-why do they like them?!)
I wrapped books, colouring sets, puzzles, a mini snow globe, sticker books, mini star wars figures, spy sets, sunglasses, singing toothbrushes, mini etchasketch, squishy eyeballs, farting slime(v bad idea that one...), all sorts of junk.
It worked coming back too.
Also, we were on the plane last, and the air hostess noticed there was an empty seat on the row behind us and shuffled everyone around.It was worth it though, and they shuffled on the way back(2 spare seats, baby laid down on her own seat too!)too, so I imagine they do try for you where they can.