I haven't taken kids on a plane before, but used to travel a lot when I was a kid, with my parents. My Mum used to make us up a little bag of treats (with everything from sweets, mine always had a new barbie, books, cheap little games etc) and we knew we'd only be allowed them if we behaved up until the seat belt lights went off.....I used to spend so much time watching the sign and waiting, that I didn't even think about wriggling around or anything! And when the sign did eventually go off it was like Christmas!! ha ha! As a grown up I can see what a neat trick this was and will definitely use it on my kids in the future!!
Good luck and hope you have a lovely holiday!!