I don't have kids, so I've never seen this experience in real life. I do have a dog though, and when I went through a car wash with her in the car, she just seemed angry. Lots of barking and confusion.

The parent who filmed this video explained in the description that they tried to prepare her for the wash before hand. They hoped she would not be scared, but it didn't work. Luckily, as we can see in the video, by the end she seems pretty happy!