You could consider looking into an area like Mui Wo or Pui O. They're beautiful areas and actually, in my opinion, potentially more livable than the big city anyway (kind of like the countryside). There is an expat community there, so I don't think you'd feel to far away. I'm not completely sure, but I would think one of these village houses that's tucked away from good transportation links (which isn't too much of a concern for you) would be within your budget. You can look at the prices of the international school there, Lantau International School. You wouldn't by chance have any contact here who could help you set up by browsing local ads for village houses, do you?
Personally, as tough as it might be, I do think children can benefit greatly from the experience of living abroad, which would open their horizons. Learning a second language (esp. natively) will help their cognitive development and thereby other skills. It's a fun language.