I don't have this experience, but I guess I can share my experience growing up in a foreign country.
My family moved to to Venezuela when I was just 3 year old. My parents never learned Spanish (my dad speaks a little bit, and my mum only learned to say very basic things, like numbers). We were all placed in local schools (there weren't any Chinese school back then, and international school would have probably been too expensive for us). As I was just 3 year old, I could get accustomed very easily... My brothers however, who were 9 and 11 at that time, did have to struggle a lot with the language.
I think what really helped us catch up with the language was the fact that we were really surrounded by it, and we really use it everyday... We didn't have many Chinese friends, and even the Chinese friends (kids) that we had would try to speak Spanish with us. We watched TV, and the programs were all in Spanish. We went out, and every sign was in Spanish.
I think if you really want your kid to learn Chinese, in addition to putting him/her in a Chinese medium school, you have to really encourage him/her to use it. For example, instead of watching English program, encourage him/her to watch local Cantonese programs.... Take you kids out, and let him/her play with local kids, etc...