Oh, Ayesha! You are a brave woman! We live in New Jersey, and most of the time, we drive down to Walt Disney World, so we've learned a few tips and tricks that help us make the journey a little less painful.
If there are two drivers, we try to leave late in the day - like dinner time. We buy meals for the kids, they play with the toys included with the meals, and by about 8 or 9 o'clock, they are sleeping. One of the drivers goes to sleep with the kids, while the other drives into the early morning - about one AM. The other one of us will wake up and continue the drive until morning, when the kids get up for breakfast, then we switch drivers again to head the rest of the way down to Florida.
If it's not possible for us to do the trip overnight, we always have a portable DVD player and a supply of movies with us. I try to have at least one or two new movies on hand. You can sometimes find old cartoons on DVD for a dollar at the dollar stores, but WalMart always has a selection of $5 movies that I can go to in a pinch. I also like to keep a few new toys in a bag, things that are new and different and will hold their interest for a while.
For the baby, you might want to plan your meal stops around her as best as you can. At two months, she probably still needs to be fed every few hours, so that's a good time to make potty stops for your son, give him a chance to stretch his legs and wander around, and grab a bite to eat or a drink.
We also try to stop in shopping areas as opposed to rest areas. There's nothing wrong with a rest area stop, but if you stop near a shopping center - especially one with a WalMart or Target - it not only gives your six year old more room to walk around, but it also allows you to pick up a snack or some drinks that won't break the family budget; and you might also find you can grab a small "reward" toy for your son's good behavior on such a long trip.
Have a great time! You'll do just fine on the road!