I know some people that whose child had asthma in hkg and I know of others who had asthma from natural sources back in oz but who were fine in hkg.
So I guess it depends. And where you live in hkg will affect how much air pollution you will get. I think some of the highest readings are from tung chung which has no industry. So if you move to somewhere that is shielded from china air should be better. Maybe DB or south side of hkg island.
Kids will spend a lot of their day in school or in home. SO if you buy air purifiers for your home that will cover up to 16hrs per day.
LA, NY, Syd, Mel, Lon etc all have varying degrees of air pollution. I realise we are worse here, but significantly less than beijing and shanghai.
Weigh it up against contact time with your kids. Better to have kids here whilst they are young than older.
cheers