I was replying almost exclusively to this:
Which is false (in a general sense).
It is mistreatment because of the effects of the (specifically cigarette) smoke. I wasn't actually hitting the legality argument at all since it's not illegal to smoke cigarettes in front of your kids.
Secondhand smoke is (really) bad for kids. The literal definition of child maltreatment is "Intentional harm to or avoidable endangerment of anyone under 18 years of age." This objectively declares someone that smokes cigarettes in front of their kids as child maltreatment, I'm not just blowing smoke (har har). Maybe you're confusing this with child abuse: "deliberate action that is harmful to a child's physical, emotional, or sexual well-being." Or child neglect: "failure to meet a child's basic physical, educational, or emotional needs".
Whether smoking is child abuse or child neglect is up to interpretation, but it is objectively child maltreatment.
Parents are allowed to fulfill their own desires, but I think my description of the act in front of kids as selfish is accurate. Definition of selfishness: "It is the act of placing one's own needs or desires above the needs or desires of others."
Maybe I should clarify clarify the label as a "bad parent." Your argument is that smoking in front of kids has little relationship with either being a bad parent or a good parent.
Parenting actions can probably be classified into three categories: good action, neutral action, and bad action. Smoking is definitely not a good action. Your argument is that it is closer to a neutral action than a bad action. This I disagree with. In the past the effects of the smoke were perhaps not as clear so this lends a reason to why it was socially acceptable. Today the effects are very clear, but unfortunately people are still addicted to these substances and it's not easy for people to just quit.
I agree that smoking in front of your kids is only one variable in determining overall parental quality; that a smoker can be a better parent than a non-smoker that does other things wrong. But I believe the action itself very clearly is lumped into the category with which a bad parent would do. So while calling someone a bad parent based upon this one tidbit of knowledge alone may be a slight exaggeration, because of the seriousness of the irreversible damage it causes I think it is appropriate.
My issue is more with the child not having a choice in the matter. I believe smoking in public should be illegal everywhere. When I'm downtown at practically every corner I see parents with a kid in their left hand and a cigarette in their right. As an adult, I can simply walk away and go somewhere else where people are not smoking, or avoid areas in general where smoking occurs. The child doesn't have this choice.
I don't know if it necessarily warrants taking a child away, but if the alternative is doing nothing then I would be okay with that. The best solution is probably somewhere in the middle.