-overall air quality is the same everywhere, but you can make things better or worse for yourself depending on whether you'll need to be spending much time along busy roads breathing in car exhaust.
-I find that HK, overall, is very family-friendly. At least compared to other places I've lived (mostly US). Pretty much every HK neighborhood with a lot of people living there is going to have a ton of kids around, and a ton of grandparents too. Interactions btwn locals and westerners could vary a lot though.
-70K is a huge budget. enjoy. I really like our place, and wow is it cheaper than 70k. tbh i'm not even sure what I'd do w/that much to spend on rent.