Our first time driving down was with an almost 5 year old.

Are you driving in a minivan?
This may sound silly, but the very bestest things we bought were one, the over head clam shell thing for our luggage and two, a portable potty. We got it at Bass Pro Shop for less than a $100 in 2006. Put up two sticky hooks on the back window, made a piece of cloth into a curtain with circle{shower} clips on it for privacy and removability.

I swear our youngest spent most of the trip going potty. Not having to look for a stopping place when "I have to go potty" was announced, priceless. We used it again in 2008 and plan on taking it with us this time.

I though my DH was mad/crazy for wanting it. We took an hour out of trip to go get it the first time. But I thanked him over and over and over again for having that in our vehicle.
PS-it has also been great for when the power goes out. Have a cistern, no power, no water.
Have a great trip!!