That's true. I brought C back to HK last year and asked the same question to HK custom. Even we have HK ID but if the kid was born oversea, she has to apply for resident first. And then if she lived in HK for 7 years, then she would become HK perm. resident.
you can ask your relative in HK to send the application and ask your relative as a representative. The immigration officer will contact your representative.
They will ask you to give the documents to prove your bb has 居港權. Once they approve, they will ask you to have interview and see all the document in original copy (as you only gave him photocopies via fax from Canada).
If you cannot attend the interview, you can tell them to postpone until you arrive to HK (of course, you have to give them a date)
If you are the canadian citizen when the BB was born. BB may not able to get the 居港權.