Put an entire outfit (including undies and hair accessories) in a ziploc bag. One bag per day. When you get to the hotel, put the bags in a drawer and they can pick one per day. Did this for Disney trip. Put dirty clothes in bag at the end of the day.
10 ways to keep kids happy in the car on a road trip. A couple of these were new to me--like having your kid color a paper car and moving it along a ribbon strung up in the car to visually show your progress along your route.
The Fun Cheap or Free Queen: How to survive a road trip with two babies by yourself.roadtrip tricks, tips, and creative ideas! (plus link within the site to more tips from a trip w/ slightly older kids)