Been there, done that, as a kid. I was the youngest, so I was crammed in the middle (my sisters were teenagers), and my baby brother (legal at the time & didn't know how unsafe) rode in the front with my parents. It was from Chicago to WDW--it was miserable. On the way down though no one cared, hey we were going to WDW!
In just 14 days we're (me--one week shy of 6 months pregnant, dh, DS4, and DS15 months) are driving down in a small vehicle (subaru Forester) from just north of Houston. It's my DH's company car & they pay for the mileage, any problems, and gas. Free trip down this way. Luggage is going on the top in a carrier. We would usually take my larger SUV, but you make do with what you have.
My suggestion is just to stop often and stretch--it makes the trip way more enjoyable. And bring things to occupy the kids (DVD, books, coloring books, snacks, etc).