Bekybook44, I thought you did a great job. You have taken into account how the different members of this site feel, with their different statuses here, ex-pat, local, foreigner now calling HK home.
One thing, I'd make it shorter. I'd take out the things they can do. They know what they can do and they've already had quite a few think thanks on the situation that say all that and more.
I would also say that the letter is being written not just for the sake of our children but for our health and the health of all people living in HK. We aren't martyrs and we don't need to be. We have a right to breathe air that will not harm our health.
Perhaps we should actually think about a class action lawsuit. If there are known and easy measures to take that can reduce pollution but the government isn't doing it then i'd say we have grounds for a lawsuit. Woodk6 maybe you can advise on this? It doesn't have to be real, just the mention of it might get people to listen a bit harder and think about their rights a bit more