Put a plastic sheet over the car seat to protect against nappy leaks
Bring way more towels and paper towels and wipes than you think you need
Leave space next to the car seat so one of you can sit next to them if needed
Leave twice as much time to get to your destination as Google maps says since you'll need to stop all the time
Have a couple of brand new toys that are nothing like they've seen before so they're more likely to stay amused for longer
Have some CDs of familiar lullabies or other music that they like