Well - the airlines seem to always have a pack of something to try and amuse children. Not sure how good they are? I would say pack a special bag of "stuff", containing say 8 different activites (one thing each LONG hour!!) several books, colouring pads, paper, crayons, gameboys (borrow one if necessary!), travel board games, snacks, and a real treat (perhaps a holiday item, like sunglasses, or a toy, or whatever floats their boat) that you can tell them you've got so that if they've been good for the entire journey, they can have it at the baggage claim!!! Perhaps an atlas book, that they can find their destination on, and a tour guide, that they can read through to see what things they'd like to do the other end. Are they good at being set tasks like this?
Find out in advance whether the airline would allow them a tour of the cockpit (I know I know, we're all terrified of terrorists, but two children? Are we THAT mean now?). Or perhaps ask the hostesses, whether they can show the children the place where they make the food (CLUTCHING CLUTCHING at straws here!!!)
Remember that we ALL use the "waiting for the food to arrive" trick to waste the first hour or so of the journey, there's the long haul bag of "goodies", like toothbrushes and such. You can spend a good 15 mins taking them to the loo to brush teeth etc. Get them to write a few postcards
How about sitting them next to an old lady (preferably American, for the novelty factor), as they always seem to have all the time in the world for children, and could maybe tell them all about where they're going! (nb. to that lady - "I'm sorry"!!!!)
Consume all the free booze on offer, and make sure your kids are checked in on seats geographically on the opposite side of the plane to yours!!!
Otherwise, I'm with the drugs option!