J.V.
I have traveled alone with my 2 girls flying for 4 hours and driving from San Diego to Vegas. I found several things to help make the trips easier
* pack a kids backpack with smallish toys (airplanes, cars, dinosaurs, horses, dolls)
* small aquadoodle cards or pads (no worries if she sucks some water or gets it on herself & the car)
* NEW DVDs of characters she loves (my girls love Diego because of the animals, Baby Einstein, and My Little Pony, Aristocats, Blue's Clues, Wonder Pets)
* Snack packs of things she can feed herself (don't even worry about the crumbs on the floor) cheerios, Gerber fruit nuggets, animal crackers may be a bit more messy, soppy cups of water & juice.
* sing songs with them: wheels on the bus, itsy bitsy spider, slippery fish, old Macdonald, 5 Little Ducks, etc...
* enlist your 3 year old's help to entertain & hand your little one new toys from the backpack.
* plan to stop frequently so they can run & stretch their legs (a porta potty (potette) has been a lifesaver if your older daughter is potty trained and their are no potties in sight/for miles.
Good luck!