Should we apply for a a China visa for our child or a travel document?

Hi,

Our child was born in the USA. I am a Hong Kong permanent resident and my wife holds a China passport. Neither of us has USA passport of green card. If we apply for a viza for our child, could we apply for it through one of the travel agents in the USA while we are in Hong Kong? What documents should we submit for the applcation? Appreciate it very much your feedback on this matter.