Use a carrier at all times when they get older- When they are younger kittens can be held in the car if there is more than one person. However as they get older, stronger, and more “risk taking” it’s just not worth the hassle of constantly trying to pin them down. Not to mention it’s simply not safe. When they are no longer kittens, use a carrier at all times in the car.
Thanks to Petmd for these suggestions