I would like to say we should handle it very very very carefully. Bilingual children may experience speech and language delays (Re: It is not a must!!). If a child has been exposed to more than one language, extra encouragement should be used to increase receptive and expressive language skills. If a child is diagnosed with a disorder such as problems processing the language, bilingual enviornment definitely jeopardize the chances of recovery. Contrary, someone says bilingual children can catch up around K3 (not sure, poor memory) ......
BTW, I am not sure if the story of Carl Weter is 100% truth (I did google Carl Weter) but I am not going to say educating a child in such way is wrong.
Whatever you do don't force a foreign language onto your kids...
I allow my kids to progress naturally when it come language simply because the environment is critical to successful to learning a language. Cantonese is normally spoken at home and outside hence this is first language we use.
My boy just 3 yrs old is more receptive to languages then my girl, 5 yrs old. My boy can speak in full sentences in English by watching English children TV and off course speak full complicated sentences in Cantonese (probably picked up from sister). My girl English although good as a second language but not as good as my boy's. Hence, I speak more in English with my boy. Each kid will be different.