Hi M.,
I am a physical therapist who specializes in vestibular disorders (problems with vertigo, dizziness, motion sickness, balance). Motion sickness can be normal in children. Their neurological system is not yet fully developed and they may have a hard time integrating their moving visual environment. Not to mention kids in the back seats usually cannot see the road which "confuses" the visual/vestibular system. Have your daughter focus on something that doesn't move, crack the window so she can have some fresh air and try to find a car seat that will elevate her enough to possibly see the road. My daughter had a mirror on the back of the seat in front of her chair. She was able to watch herself which helps reinforce her visual system input.
The vestibular organ is located in the inner ear and is one of the 3 main senses responsible for balance. The other 2 are vision (integration) and somatosensory (info from body movement sent to your brain by nerve receptors in joints, muscled tendons, etc). I would recommend seeing a health care professional who specializes in vestibular disorders. Don't go to just any doctor. They will not know or be familiar enough with the vestibular system (or even open to the recommendation. You will need a referral prescription from your doctor however. In San Diego I found the following on www.vestibular.org:
San Diego (Rancho Bernardo):
Judith K. Horning, AuD, MA, CCC-A
Rancho Bernardo Hearing Services
16766 Bernardo Center Dr., Ste. 206
San Diego, CA 92128
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The Audiologist will do testing to see if there is any "weakness" with the vestibular system. She can also refer you to a pediatric or vestibular physical therapist in your area who specializes in vestibular rehabilitation as well as a doctor who does.
Since your daughter is so very senstive to motion (maybe a little more than usual with kids) that is what I recommend.
Good luck