Ok.....I will tell you what we do because we always drive these days with our family of 6. We did fly one time when we were still a family of four but that was 5+years ago!!!
I always pack a medium sized hard sided cooler that I fill with bottled water, juice boxes, cold meat/sandwhich fixin's, and fresh fruits such as grapes, apples......very portable. We then make stops at rest stops along the way so the kids can stretch and run/play and we do a picnic kind of thing. We are driving from Indiana so we have found once we clear Tennessee(sp) the rst stops are very clean and very nicely maintained. I pack a ball/frisbee for play when we make stops. We also dont drive more than 3 hours before stopping to let the kids stretch/potty. Having said that we leave early in the mornings, around 4-5am and drive about 5 hours before our first stop. Or next stop after that would be lunch.
I only pack what we will need for the trip itself because we make a grocery run after we have arrived and checked in/unpacked.
We have a DVD player in our truck so we bring every Disney movie we own, i have a boxed CD set of disney music we listen to and we play road games as we go. Last trip to Disney I made the kids ctiviy books with print outs from the internet and bound them in a three ring folder. Also each kid packs a backpack full of entertainment. My oldest packs lots of books, her ipod, journal etc. My oldest Ds packs art supplies,
lego/transformers, his gameboy, etc. We pack color wonder by crayola for our youngest DS, some matchbox sized toys, etc.
I always pack the take n toss sippy cups to reduce spills and they are great in the parks. I also pack a couple things of antibactrial wipes to keep dirty sticky faces and hands clean. Baby wipes are ok but I prefer the antibacterial for this purpose. Having two of our four in diapers still i always have a stash of wipes, HA!
Also pack a small first aid kit. I use a soft sided cooler and stuff it with tylenol/motrin, antacid, pepto bismol tabs, band aids, neosporin, claritan-d and benadryl. All of those things are pricey at WDW and you never know what you will need to doctor on the way down. Also my DH is type 2diabetic so I keep copies of his prescrips and a business card of each of his, the kids our family dentist and my DR. in my wallet. You just never know when you will have a medical situation and by having these on hand I can relate the info right to emergeny personal or make a call home to our DR's with questions.
I dont know where you are driving from or the ages of your kids so I cant give you better info than this but I HTH!!!
Have a great trip!