small games---Target sells travel sized games, coloring books and crayons, blank paper and pencil/pen, those invisible ink activity books (they love those), cool activity books, sticker books, cards (for the younger ones you could play go-fish), legos or a transformer type toy...something they can build or something, paper dolls, those school workbooks or sheets, mazes and puzzles and word searches printed off the internet.... just about anything I think the kids would like. I recommend buying new ones, which will be more exciting. And I mentioned on another thread, if you think your kids might still get antsy, wrap a couple inexpensive 'presents' for them to open. Bring a variety of snacks, too, and a pillow, blanket and stuffed animal.
The other PP said the least amount, the better. I'm the opposite...I bring extra stuff....just in case, and I've never had a problem with my kids on a plane because of it. It's worth me lugging around a little extra stuff, as long as we can fly in peace....which is priceless. Good luck, and have a nice trip.