We jsut flew to Mexico with DS who was 3 in October. We took colouring stuff, a book on steam trains (so a book on something your DC is interested in), silly toys (like stuff off of magazines, plastic snakes etc etc). Oh and food. I had so much stuff, a whole sports bag full of stuff, and we did not need all of it. He could watch the kids channel on the TV and eat and look at his book he was fine
HI, flew to washington last sept with our ds. he was younger and i asked for lots of advice too. most said, don't take too many small toys as you won't have space to store them and you might lose them anyway. books are best plus the kids channel will be fab and maybe some sticker books as they're easy to transport. you'll probably also be given some kind of kids pack with colouring books in by the airline.
We flew with DD to Florida last yea when she was 2yo. DD took a small flight bag, with her flight toys in. It had her teddy, some magazines, some books, crayons and paper, stickers, a magna doodle type thing, a little car, etc. We also put in some snacks - raisins, juice cartons, etc.
On the flight she enjoyed havinbg a walk, trying out the toilet (!) and watching Shrek (although struggled with headphones). Looking out the windows is a bit boring though as flying over sea mpst of time.
Have to say that is was actaully way easier than we had imagines.
Considering flying to Australia next year - DD will be 4.5yo - eek!