I really feel for you. We went through exactly the same thing a few years ago and it made our lives a nightmare. Our little girl was a toddler at the time, and the woman who lived downstairs at the time was a heavy smoker, and although it started off as just "normal" smoke (which sometimes caused a haze in our living room and kitchen), we started getting an increasingly strong cannabis smell coming through. We started off by just mentioning to her that we had a problem with smoke coming through, in the hope that she would realise that we had noticed the smell, but it made no difference. As far as she was concerned, she owned her house and could do what she liked in it. Which is fair enough, except that what she was doing didn't stay in her house! The constant smell of it made me feel sick, and on one memorable occasion, my daughter was running around the house, arms waving, acting as though she was high. That was the last straw. I called in Environmental Health at Wandsworth council and the local police, who were both very helpful. They took the complaint seriously, especially because a child's health was involved. For us, unfortunately, the problem was never fully resolved, and, as we were renting, we just moved out. However, if we had stayed and continued fighting it, I don't doubt that between Environmental Health and the police, they would have got it sorted out. If she had been Housing Association, it would have been dealt with even quicker, because we could have complained to them, too.
Don't feel guilty about kicking up a stink. It's your child's health at stake, and the nasty, second hand chemicals do have an effect! Good luck x