I don't really have a clear view of this, it's just my opinion was before the PRC took over HK in 1997, there is no real way to determine whether one is a PRC citizen or not. The language in one of the interpretations is that those that have 中國血統 (i.e. Chinese race/blood) will be taken as Chinese citizens, at least at the change of sovereignty. As for how 中國血統 can be interpreted, that beats me.
I do not know how they treat this in practice for any newborns in Hong Kong now. Perhaps if they look Chinese and you claim them as Chinese under the nationality field they'll just take it as that. I have no idea.
The concept of "nationality" in the Hong Kong perspective is very clouded historically, I don't think people there have a grasp of that. They used to be administered by the British, were not part of UK, not full British citizens, most residents are Han Chinese, returned to China in 1997, etc. What also complicates is that under PRC and HKSAR policy, the Hong Kong territory always has been part of China, so they interpret things in that it was always part of China, just under British administration for 150 years. In a legal perspective, it's like China considers that its law ruled over Hong Kong even when they weren't in control of it.