When traveling with my DD (she has done lots of LONG plane trips since she was 6), I do the following:
She has her own fanny pack - in which she carries her emergency identification info (in case we should get separated), a little bit of cash, a snack food such as a granola bar or raisins, her color game boy with fully charged batteries, a small notepad and a pen. In addition, she brings a book that she is interested in and has not read yet.
She stays very entertained this way. She reads, she doodles on her notepad, she plays her gameboy, and if she is starving - she eats something! It always helps if they have an in flight movie that is reasonably appropriate for her as well.
As for me, I carry in a small bag my passport, tickets, wallet with ID, credit cards, & money/travelers checks, digital camera, and my medication. I also carry a good book with me to read and a light sweater jacket. The rest I check. There is nothing worse than trying to hang on to a child, and a bunch of your possessions while navigating around strange surroundings. If you have your medications and essentials - you can purchase everything else in a pinch.