I must say the Belair building and facilities are lovely and the views can be quite enticing. However, like most of the newer buildings in HK, the flats are small - something advertised as 1500 sq feet will be more like 1100 and you may find you need one of the bedroom just to store items. Another consideration is that it is a bit far from central, where many people work. As Sapphire noted, the transport is quite good but you might find it a bit more work to venture out of the area (or find that you don't). Then again, the dog and little one will enjoy the nice open park area and there are TONS of families that live there so I'm sure you'll quickly meet people. Guess it comes down to how often you want to be able to go to Soho for dinner, LFK for drinks, etc. The other thing to remember is that a lot of the children's classes/activities may not be available in Belair (think sports, art, etc) which means you'll either want a car, spend a lot of time on the buses, or spend a good deal on taxis.
As the others noted, I'm sure you'll see other places and areas when you arrive and who knows how your views of Belair will or won't change. . . .