I'd advise against it. There's a non-zero chance that a character or artist could go by "nymphet" in the future, and I doubt anyone will do a search for "nymphet" looking for Kodomo no Jikan.
I'd rather not. We don't need to recognize an incredibly dumb "translation" that never went into actual publication, especially since it's just a single normal word.
Would it be okay to add a note about that title to the wiki, then? I agree, there's probably not too many folk out there who'd be looking for this and not know the Japanese name, but at least the wiki search could point them in the right direction.
