I have thought of this before. After visiting my friends and talking to their kids in Beijing, I have changed my mind:
1. It is super hot in summer time.
2. The nannies cannot speak proper PTH.
3. The kids don't have many opportunities to speak PTH. If you put them in school/courses, probably these would be dedicated to foreigners including Hongkongers. If you go out with them, it's more likely that parents will do the talking.
My friends' kids are studying at international schools. They say that the only chances to speak PTH is during Chinese lessons (not many a week), talk to nanny (limited) and drivers (very limited). They won't watch local TV (not interested).
How about Taipei? Will it be a better alternative?
I had a PTH teacher come to our home twice a week, not expensive. My kid study around 6- 7 mths, she like PTH very much. Every lesson, she will learn 拼音, reading , do some exercise such as matching the chinese word to the sound 拼音, write the 拼音 beside the simple chinese word. sometime singing, poem ...etc . very happy and learn many PTH. Sometime they will see the PTH dvd at home. I think hire individaual teacher is more effective !!