You refuted yourself in the second paragraph. There is no reasonable definition of human that would apply to a child but not its parent for the reason given. Human is a vague predicate. You'd need a definition in which a single mutation across a generation allows a nonhuman parent to have a human offspring, and you can't do that.at some point a non human (someone that doesn’t fit your definition of human) gave birth to a human ](someone that fits your definition of human)
The problem is that for any practical purpose it is hard to draw a line between human and not human…………..as an analogy an young man will eventually become an old man………. But you can´t really draw a line and point to a specific day where that happened
The sorites paradox examines this. With which loss of a single hair did a man become bald? At what minute did his beard become a beard? At which moment did the first light of dawn appear this morning? None of these can be answered precisely because we don't have definitions of any of these things that are that so precise that we can identify exactly when the transformation occurred.