I have 2 ages to consider, one boy of 8 and a girl of nearly 3 (and one big kid of 34!) my son will always pick what he wants to put n his bag, usually consists of DS, book, puzzle book and a pad and pen. My daughter I always take son's old GameBoy, stickers (lots of them), crayons, colouring books, small picture books. The last 2 flights we've been on to the UK have been night flights and they have slept most of the way, or watched the films. A suggestion to me was to take something new, so if they do get a bit tetchy, usually seeing a new toy takes the edge of it a bit!
Don't know if this helps!