T.C.
It sounds like it will be a memorable family trip! I haven't been to Disney World, but we just got back from a trip driving from Texas to Disney Land in California.
My husband always jokes that I pack everything, including the kitchen sink. For this trip, I brought my own pillow to be comfortable, a small blanket if I got cold in the car, a towel to keep the floor of the rental car clean from sandy shoes and drink spills, a cooler full of bottled water (water bottles with a lid were allowed at disneyland and we could refill them from the water fountains), trail mix and other snacks for the trip, plastic dishes and some food to cook if the hotel had a microwave, swimsuits, sunblock, hand sanitizer, basic medicines including a thermometer and remedies for carsickness and constipation, hats, camera with extra batteries, cell phones in case we got separated at disneyland(don't forget the chargers), ziplock bags to keep things dry if we went somewhere with water rides, enough clothes to do laundry halfway through the trip, breathable socks and 2 pairs of shoes for each of us; small toys, craft projects, books, and music CDs for my son; and for my husband- GPS and a folder with printouts of all the info he found ahead of time- hotel info, car rental papers, family and emergency phone numbers, cheapest gas price locations, local restaurant coupons, maps and travel guides. I hope this gives you some ideas.