Car sickness is most common between the ages of two and twelve, and it happens because what your child's eyes see and what their inner ear feels do not agree with each other.
Seating helps more than most people expect. A child who can see out of the front windscreen gets far more agreement between those two signals than one looking down at a book or a screen.
Air matters. A slightly open window or good ventilation reduces symptoms noticeably, and a stuffy warm car makes them worse quite quickly.
Light food before travelling is better than either an empty stomach or a heavy meal. Both extremes tend to make things harder.
And have a plan for when it happens anyway, because sometimes it does. Being ready turns it into a two minute pause rather than the event that ruins the trip.