The vehicle and seating position can make all the difference. They need to be able to see and focus a distance down the road. If their focus is close to, or inside the vehicle, they will feel all the movements exaggerated causing nausea.
My sons (12,14) get car sick when in the back seat of my Honda Accord. It has a tall beltline and low rear seats making outward visibility more difficult. My wife says it's my driving but they don't get nauseous sitting in the front seat. They don't get sick riding in my truck (front or back)either.
Depending on age try different seating positions. Up front, taller booster etc. Also, feed them. Like flying etc, don't go on an empty stomach. People are afraid to eat because they think it will make them sicker but you are actually less likely to feel ill if you have something in your stomach.
If you are towing it may have something to do with the vehicle and trailer combo. Better shocks and aired up tires can stop the wallowing movements and a better hitch can stop sway movements. The guy I bought my first Hensley hitch from bought it because his daughter got car sick when towing their trailer. She was fine once he used the Hensley.