From my own experience, a person can be completely fluent in more than one language. I became bilingual in much the same way as Howard's kids (via total immersion) and I have no recollection of struggling. However, my second language (English) is now my dominant language, while my first langauge is better than functional. And I do not have any special talent for languages.
I find it amazing how quickly kids can pick up languages and I feel that there is a great window of opportunity to teach them while they are young. After a certain age it seems too difficult to reach a native proficiency level - the accent of the dominant language prevails.
My husband and I are fluent in two different languages + English, so it would be a crying shame not to pass them onto our child. But the trick I think is to make it fun and do what the child enjoys, and the learning will follow. Also, support the language with as much outside exposure to clear native speakers as possible (courses/clubs/activities) as Howard suggests. This may correct pronunciation/grammar issues that one often hears, for example, when listening to a Chinese parent speaking broken/terrible English to their child.
For us, the problem with the local Cantonese Kindy, was the manner in which our son was taught and of course the teacher could not adapt to one child's needs in a full class of 25 Cantonese speakers. Yet this same child is having no trouble picking up German just through playing with his dad!
Howard, I certainly salute your efforts. You are raising multi-lingual international kids!
I just wish the HK education department would make Cantonese more accessible to everyone rather than promoting a segregated society.
My only hope now is for a bilingual (mandarin) International school... shame, because our chid could have been more assimilated in HK.