K.I.
Traveling at night is the best way! The DVD player and movie is also very helpful, as well as any "gameboy, DS, PSP, leapster" hand held devices, they are so cool and you can find the older versions and used games fairly cheap at the local "trade a game" type places.
I always dress them in comfy clothes, bring their pillow and blanket and try to make them as comfy cozy as possible.
Snacks are so easy...especially if you want to pack a small cooler to fit in front with you...you can make sandwiches, pack individual snacks in baggies, ready to be doled out as necessary. Mini donuts are a fun treat for my kids, they love them! Spill proof sippy cups are also a life savor on the upholstery, even for the 4 yr old. You can pack one big thing of water/juice/gatorade and fill as needed.
When we travel in the car like this...all bets are off as far as "normal rules" apply! I usually pack them a healthy sandwich, then go crazy with all the goodies they normally never have! Keeps it fun, and keeps everyone in a good mood!
I have my own 12 hour car ride to plan for this Aug. I will be traveling with 5 kids ages 18,15,6,4,2...gotta be thankful for the hubbies who will drive thru the night to make things easier for the rest of the family!!
~Don't forget the trash bag for all the cleanup...saves the poor car from getting thrashed from sticky chocolaty wrappers...and the extra diaper wipes for lil' hands and faces!
We always do the books on tape too...although your kids are pretty small, maybe you and your DH could pick something for you two to enjoy for once the kids fall out!
Have fun and make it as comfy cozy as possible! It'll be great!