I always pack a "baby"/child blanket in my backpack as quite often the airplane blankies can be scratchy. And then they come in so much handy for other uses as well- something to sit on when out and about, in the pram, blocking out light when necessary etc.

I also pack a few small food items that I know my kids will definitely eat and that may not be available easily elsewhere e.g. vegemite, crackers etc. This is not just for the plane but for extended holidays.