What will your kids sit and do?
The older two should be able to pack their own bags. My DD's 6, and she's responsible for packing a backpack that she will carry, but I have final say on things.
Her bag generally ends up with: craft kit (plain paper, twistables, stickers, colored post-it-notes, scissors, tape), coloring book, books, Dolly (comfort item) and a few outfits, pet shops, nintendoDS (used to be leapster)/headphones. I edit down the number of books and the pet/accessories choices, then add a workbook and/or BrainQuest.
I'll carry a new book, one other toy/craft surprise (for emergencies) and healthy snacks, though part of our ritual is that she does get to pick a bag of candy and a beverage at the airport. Again, mom has veto power over her purchases. I also load music for her onto my ipod.
For your toddler, a few favorite and new board books, crayons/coloring book, comfort item, travel magnadoodle, invisible ink book, favorite small toys. Wikki sticks/pipe cleaners, maybe. Run around the airport before you board, and don't buckle in until you have to.