When my children were small, I would stop the car if they got out of their car seat. To this day, I don't start my car till everyone is buckled in. One time I even had to tell my son he couldn't come with me if he didn't stay in his car seat (he was 2 and liked to climb out).
A very long time ago, (1982ish) I was asked by a roommate to go with her to Utah's Primary Children's Hospital. A friend of hers had phoned from Idaho. She was in the hospital in pretty bad shape. She had been driving along with her 4 month old baby boy in her lap when along came someone and hit her head on. She crushed her little boy between herself and the wheel. She was desperate because she couldn't be released to go see her son who had been life flighted to PCH.
We went. It was one of those moments in my life that I don't often choose to think about. There lay in this incubator, this tiny little baby boy who couldn't move. The nurse told us that he wouldn't be able to move any of his body, he was paralyzed. They said he had a good chance of surviving but didn't know if he would even be able to speak. I never forgot that tiny little boy so broken.
So I'm sorry, but if someone's riding in my car and won't stay put, parent or not, I'll swat them if I've been given charge of them. I'd rather swat them than have them hurt should someone hit me.