At age 3, when running outside and playing in the grass isn't quite as important, you'll probably be better off looking for "kid friendly buildings" rather than a specific area of Hong Kong. One of the first things you should look for is an indoor childrens' play room in the building. Many of the buildings have these facilities and they are amazing for meeting other kids, letting off steam and providing you a place where you can safely send the kid for a few hours with a helper if you need some time to work at home. Some buildings have small rooms, sufficient for a half dozen toddlers, others have absolutely insane places with nets, climbing towers, padded walls, slides, etc.
Add to that look for somewhere with an outdoor facility as well, and possibly even a pool. A place with a function room also serves as a low-cost birthday party room where you can socialize with other parents in your building.
As for more of an area question, have you started considering schools yet (not pre-schools--schools). If you have your heart set on a particular school in one corner or the other of Hong Kong, or have a debenture that will ensure admission to that school, you may want to consider living closer rather than farther away. For example if you have a German passport and wish to attend GSIS kindergarten and primary, than you might want to live near Pok Fu Lam where that campus is located. Or if you are choosing the Aussie school you might consider locations near that.
If your child was a year older, our advice (seriously) would be 'schools first--then a house'. You have a year before that all gets underway but you might be thinking along those lines.
Oh, as for air pollution...ummm...
You'll hear from real estate agents that the air is "better on the South Side" or in this corner of the New Territories or that one. However, there are a limited number of monitoring stations run by the government in HK and none on the South Side. In addition, that part of HK is a) close to the shipping lanes and b) close to a coal-fired power plant on Lamma Island. And when the winds blow in from China's factories it can be bad all over. While it doesn't have the massive number of cars in Central, the air down there isn't perfect, not by a long shot.
Here is a map of the air pollution levels tonight. Everywhere in red is over 100% of the World Health Organization maximums (Causeway Bay is 295% of the max at the moment). Note even that tiny monitoring station out on an island in the upper corner is red. You'll see there is no station on the South Side, but I'd be shocked to see it meet the standards right now when the rest of the territory is in such shape.
Is it "better" than the others parts that are 'really bad'? The answer is "well, maybe" or even "probably" but until you see the testing data don't rely on the sales agents saying "well, the sky looks bluer down here".
Basically, when it rains all over Hong Kong most people get wet. Same is true with the air pollution. When it's bad, it can be bad all over.