I agree with your idea. It's better for small children to be fully exposed to their first until they can understand and be able to express simple ideas in their mother tongue before learning a second or third language. It's useless to simply join a playgroup once a week and go back to the Cantonese speaking environment.
My son is a mixed child. I insist to have him pick up Cantonese as his mother language as I know very well I can speak Cantonese better than other languages.
When he's two years old, my husband starts speaking to him fully in Urdu, his national language. Now he can distinguish the two different language, speaking Cantonese with me and attempting to speak Urdu with my husband.
I know children at their earlier age can pick up a few language easily if they are given an extensive and balanced exposure. I plan to send him to an English/Mandarin speaking kindergarten.
My son is not so talented in speaking. I simply hope that having him immersed in a variety of native speaking environment he will have some improvement in future.