Colouring and a set of pens/crayons clean tshirt for the big one, outfit for the little one (are you trusting them or will they wear a nappy?) DS or similar; music player with earphones. Story pre-loaded on it? Small character toys but not ones with lots of little bits Storybooks Earplugs, sweets for landing, travel bands if you have a sicky one Wipes
You can take on board bottles of water that you have bought at hugely inflated prices airside.
In the DCs hand luggage we take something along the lines of:
-Nintendo DS or similar -mini-colouring books/wordsearches/puzzle books -magazine each which usually comes with a free gift (they get to chose one in the airport WHSmiths -its become part of a holiday tradition/treat as we don't normally buy them) -babywipes and tissues -snacks (lots!) -lightweight change of clothes -chewing gum for take-off and landing (but my 2 are now 7 and 11)