As a kid I used to hurl every time I was in a decent car, Jags, Mercs, leather seats and wafty suspension = buckets of chunder, spraying across the windows.
[video]https://youtu.be/ujhImZbroIg[/video]
A combo of phenergan (does that even exist nowdays, this was about 40 years ago), keeping the temperature down, looking out the front window and not looking down (no reading, even a map could bring the vom on) was the only thing i could do to reduce it.
Also, smooth driving isnt always the answer, keep it fast and loose, plenty of left foot braking which, combined with playing a car spotting game, keeps kids looking out to the horizon which may help.
Good luck.