I pack a brown paper grocery bag, one for each child, filled with dollar store stuff and other goodies. About every hour they get a small suprise or when they have been good for a while. It keeps the ride interesting and helps with behavior, sometimes they have to earn it. It may be sugar free food, fruit in tupperware, dry cereal, new crayons, books, plastic bugs or other small toys. My daughter had a strap on tray for her car seat, they sell them if yours doesn't have one. She played with small tubs of play doh, $.50 ones. If you give her a lolly pop make sure you can stop about 30 min later to wet wipe her down. I also let my kids travel in junkie comfy clothes and then they change right before or when we get to our destination. Bring some relaxing / boring music. My kids always go to sleep with Pavarotti. Don't be afraid for you and your hubby to switch drivers and also get in the back and ride with her occasionally. That really makes my kids excited.
Oh! You can go online to Chic Fil A and print driving directions on your route. It will show all the chic fil A's along the way. Not sure if anyone else does that but I call ahead to some of them and ask if their playground is indoor or outdoor. I don't do outdoor. It helps me when my kids go nuts I know 15 miles away there is a clean playground to save my sanity. Have a folder with some maps, chic fil A maps and other areas along your way the kids can stop and look around at. If you are excited they will be, for your son make it an adventure, an exploration! My son loves discovering anything and everything.
Bring a blanket so you can change her in the car. Finding a clean bathroom is like finding the holy grail. For your older one, I bought a foldable travel potty seat I keep in a zip loc bag. You unfold it and it's a potty seat so they don't have to sit on the nasty toilet seat. It's only $10 at Toys R us. I have also taken my potty seat . Mine has a cover, it is also a stool, and I keep a large bottle of water to rinse, a bottle of clorox spray to clean, and toilet paper. If they need to go ""NOW""!!! They can.
We travel A LOT. 8-10 hours is nothing for my kids. Stopping a long the way helps, it adds to you trip time, but letting them run ,makes them tired and a sleeping child on a car ride is beautiful.
Hope I helped! J.