As someone who has bad memories of car sickness as a kid, it is not related (for me) to fast acceleration/braking, so much as suspension and more gradual back/forward/back/forward (or sideways) repetitive movements (think sea sickness).
The fix, for me, was to look out of the window, which is the same as the fix for (mild) sea sickness (get on deck and focus on the horizon). If your kids are looking down at their phones/tablets etc all the time in the car, they are far more likely to get sick than if looking out the window.
Why? Because sea sickness and car sickness are a poison response (really). Your brain is continually comparing the input its gets from its balance system in the ear to what its gets from your eyes. If you ear says "rolling around" and your eyes say the same thing, that's fine. But if one says "rolling around" and the other says "stationary" that's a red flag. Often, this is a sign of eating something poisonous .. so what does the brain do? Get rid of it .. so you get nauseous to expel the (possible) poison from your body. It's the same reason you get sick when drunk, or on a roller coaster.
(I know, kinda OT, but wanted to mention it.)