Thanks for long answer.
Nox and Knox are not the same, however. (Nor are 'see' and sea' etc etc - we understand words in part from their context). I even think they sound slightly different. 'Knox' sounds more breathy to begin with - your mouth makes a wider shape to say the 'k'and that affects the sound of the word.
You are right; in Harry Potter, Nox is the name of a spell, rather than a person. It's still a name (noun).
But more concerning than these, at school, children learn about myths and legends. There are also many children's books about them, for private reading. Nox (on the whole an unpleasant, secretive, threatening character, who brought death as well as sleep) is sometimes described in them.
Children also learn about word origins. Perhaps it might not be much fun for a boy called Nox to be in a class where a teacher was explaining that the word 'nocturnal' (owls, bats etc) comes from the name of a Roman goddess.
All about Nyx (called Nox by the Romans) here:
www.theoi.com/Protogenos/Nyx.html
As others have said, Nox IS used as drug slang.
But as always, your baby, your choice. Genuinely, best wishes.