Language is the key to absorbing knowledge and it is very important for children to master good language, no matter it is English or Cantonese, as early as possible to help developing their knowledge acquisition ability.
While learn both languages shoudn't be a problem for kids, it somehow will defer the proficiency of any one language which is not so good for other learning activities. The only exception may be that the child is exposed to a mixed language environment where he or she constantly interact with native language speakers. If both parents are all speaking *native* English to their kids, the best way to learn another language is to put him to a local school and interact with local Chinese students.
I think learning putonghua is a bit tricky here since there is no pure putonghua enviroment in Hong Kong. It is possible that the kid might get confused by cantonese and putonghua since they are two very similar language and cantonese is everyday language in HK. I personally do not think it is a good idea.