It’s true that secondhand smoke comes with a lot of risks, especially for young children. Unfortunately, in a case like this, it would be very difficult to prove that your child has been damaged in a measurable way. The risks of secondhand smoke are hard to measure objectively and the medical consequences might not be apparent until years after the exposure has taken place.
If you believe that your child is in danger from secondhand smoke, the safest thing to do is remove that child from the situation. If your child is exposed to secondhand smoke in an area where no one should be smoking, like a child care center or a school, you might need to report the smoker to whoever is in charge of managing that facility.
This would be a very difficult case to litigate. A personal injury attorney would probably not take on any secondhand smoke case unless there were clear and severe injuries and a specific person or entity was clearly at fault.