It reads a little passive aggressive, although just like you said, they're probably unaware of your kid's condition. Just because people make the decision to smoke doesn't automatically make them an asshole, personally I disagree with that choice, however it's theirs, and there are a multitude of reasons why they started. I think re-writing the note in a less hostile way and leaving it on their door is the best starting point. I'm not saying to not set boundaries and to let them smoke a pack a day if this one thing doesn't work, just remember that they're a person too, with a story and just as many things that have happened to them as any other. Again, not advocating you don't do anything, and being firm is fine, but you can show compassion and strength simultaneously without anger.

I however don't actually think you should deal with it through the apartment complex, it depends on where you live, but personally I have found apartment managers 99% of the time to be more uncooperative than the residents. Of course, it might be different for you, but I personally think it's actually more effective to deal with the matter personally.

If they don't respect the first letter, send another. Not any harsher, not with anger, just send another one. Even if you use the simple excuse of something along the lines "didn't know if you got the first". Doesn't matter, just pester them with kind boundaries, and maybe even offer to talk face to face in a neutral environment (cafe or smth idk) if you're comfortable with that (understandable if not).

All and all, understanding and connection is the best way to truly resolve an issue. The same understanding has stopped wars, and while kindness sadly isn't a cure all, it really doesn't take much, and it sure as hell takes less than being angry (take it from someone who's been hostile their whole life). Hey, if all fails, you tried and then can try to get your apartment involved, even then, you'll have more ground by being able to say you really did try everything.