We pack each kid their own backpack. That way all of their stuff is in one area, separate from yours or any of your other children (if you have them). Hit up a local Dollar store a few weeks before the trip. That way if some of the stuff gets lost or forgotten on the plane, it's not a big loss. Coloring books are good if your kids are into them. Look around for the triangle shaped crayons - they don't roll off the tray table! Keep most of the new toys a secret until they get on the plane. If you're lucky, the newness of them will keep the kids happy most of the way.
I also back their own snacks in their bags - each kids gets the same as the other, but they are kept separate to avoid any issues of one kid eating the others snack.