I seem to remember that Nabokov actually invented 'nymphet' as a word in English, drawing from the ancient Greek word 'nymph' and French 'nymphe' - it became widely known after the scandal created by his book. Something like that anyway.

As for et/ette, I think 'et' is one of those de-Frenchifying things This is a pretty obscure word anyway, so people probably tend to spell it with the more normal 'ette' ending. As JJ said, 'nymphette' is used in modern French with this meaning (sexually precocious young girl, or some such). I don't know for sure whether this usage existed prior to Lolita.

I guess the most common word from this group in English everyday speech is nympho, short for 'nymphomaniac', someone who's mad about sex (it's a recognised medical condition, as well as being used as slang). It's only ever used about women in my experience, probably because it's just taken as a fact that men are mad about sex