1) They have a massive population. ~1.386 Billion people. Large enough to conceal large populations (100's of thousands of people) without drawing attention to cities, supply chain movements, etc.
2) There are no problems with using unethical or illegal actions to advance the CCP's agenda. See: parting out fully conscious and unanesthatized prisoners for organ transplants, etc.
I have no doubt that they will (if they aren't already doing it) devising experimental gene-edits and applying it to batches of hundreds or thousands of babies, and seeing what happens as they grow up. They wouldn't even need to hide them from the general population, with the whole pervasive surveillance / facial recognition / etc. thing they've got going on they could release the children into the general population and track them across their entire lives covertly.
I'm not saying that we'll end up with a supersoldier scenario or anything like that, but this could lead to a medical advantage over the west that could have tremendous implications economically and to security (the possibility of blackmailing someone into spying in exchange for medical treatement).