Whenever I go anywhere I always pack snacks that he kids have at home especially bars, raisens and that type of thing. I would suggest that each of the children (except Jack!) have a little rucksack or similar to carry onto the plane each, with stuff to keep them occupied. For Jack make sure you have change of clothes in your bag. Dylan still wets occassionally if he falls asleep (he has meds for this) and we got caught on flight home from spain as I didnt have spare trousers and it was a night flight and he wet.
I always take the milk that you need so that it is the same brand.
plenty of towels, again I always make sure I have a blanket that Erin is used too, especially for in the evenings if out in the buggy as they may go to sleep.