Hi K.,
First of all good luck to you and your family during travel times! Here are some suggestions from past travel experiences that our family has used. Lots of exercise before driving/traveling if possible. Also with driving, try driving as early in the morning as it is cooler and the kids are more apt to slowing waking up with cartoons, etc. Pack lots of snacks, water, crackers, fruit, even candy (anything to get the last few miles in). Another idea if possible, leave when they are still sleeping for a couple hours then you'll most likely have only two hours with them awake.
These are things I would also pack, lots of kid music even some simple musical instruments like a little shaker, simple whistle, sticks. You will be the entertainer if possible while your husband drives. Also books that you can read in the car/plane.
Don't forget if your driving to bring a light blanket for the kids to cover up with, as it does get cold with the air conditioner on and some thicker blankets to shade the windows from the sun by rolling them in the window.
I know this sounds crazy, but I've done it, pack some simple make-up for you 2.5 year old girl like some lip gloss with a little brush and some eye shadow, etc. Let her put some on you and on her and don't forget the little mirror. All this is fun stuff just remember to put an old shirt on her, anything to keep them busy and occupied. Don't forget you will most likely be sitting between them in the back seat, as I have in the past just to keep my sanity!
good luck with it all and let me know how it goes!
K.
(mother of two girls, ages 5.5 and 2.5)