I am sensitive to odors and have trouble dealing with a dirty car. My car gets messy, but never horrifically so. We do have quite a bit of clumpy dried dirt, salt, and sand on the floors right now, and I don't go out of my way to vacuum it out frequently....but the car doesn't smell and there is nothing rotting in it.
Everyone in our family gets carsick. A stinky car would have been hard on my kids when they were little. I would make accommodations, if only because I would not want to ride in a gross car, and I treat my children like I would want to be treated.
We have a different issue around here. Two of my close friends have children who are good friends with my kids. These two friends of mine are AWFUL drivers. Not dangerous, mind you, but they both have lead feet and the car jerks everywhere when they're driving. Stomp on the gas, stomp on the brake, stomp on the gas. You get the picture. My DS2 had to have one of these friends pull over once so he could puke out the back door. I had no idea how bad it was until I rode with her one day. We only drove about 1.5 miles down the road and my stomach was lurching. I was dreading the ride back, and I will never make either of my kids ride with her again. She lives 45 minutes away, but I will make the trip to go pick them up.