If the police can already interpret law as they choose, and secondhand smoke really is child abuse (which is the entire premise upon which the law at hand is based), then people could be pulled over on the grounds that they are abusing their children in the vehicle.
Who, precisely, defines negligence? You are saying that the secondhand smoke issue isn't negligence, obviously, because it doesn't fall under existing child abuse statutes, so why is this question even here? If it's negligence, it's already against the law! If it's not negligence, then why do we need a law on the subject?
How small and confined constitutes a hazard? Why aren't you addressing residual exposure? Is smoking with your 17-year-old in the car a bit different than smoking with your baby in it? Oh, I know, NOW the police will exercise common sense (like they couldn't when stopping someone with a coughing five-year-old in the back of their smoke-filled, windows-up vehicle). The law attempts to be one-size-fits-all, rather than enforce the laws already on the books. It's another law that doesn't do what it purports to do, and you haven't proven otherwise.
How does this law actually stop people from smoking with children in the car? How does it reduce exposure to secondhand smoke? How does it save lives?