What ages? And are they even slightly carsick? Basically they need to be amused, so if they’re not independent readers yet or reading makes them feel sick, you need to help. Screens are great and easy of course (again assuming won’t make anyone carsick), but if you want to go more old school, be prepared with music, audio books, license plate bingo, I spy, 20 questions, etc. Assume you will be carrying on kid-level conversation the whole time unless they’re absorbed in a movie/book/game.