Well...
I don't see her as appearing pre-pubsecent or childlike.
Yes, she's thin.
Yes, she's naked.
But I don't find it offensive in the way that you are - i.e. presenting a woman as "a sexually available pre-pubescent child".
Also, is not as if the advertisement is for something/anything aimed at children - if she were advertising those elastic band bracelet things or similar, then would be more questionable. But is a grown up jewellery brand. Not aimed at kids.
So based on all of this, I disagree with you that it is offensive (though do feel that a slim naked model doesn't pull me into buying the product, so failed advert from my perspective).