Well, what are you employing? A nanny? A co-hostess for the diplomatic parties you hold?
She's an au pair. She's supposed to be treated and behave like a daughter of the same age, if you had one (good luck with that, from one who's had a daughter LOL) What do you propose to do to overcome the crippling embarrassment you must feel when strangers see her in clothes which, to your shock and horror are not dressy enough for the occasion? M'dear it must absolutely ghastly for you.
I honestly can't see what the problem is. If you wanted a nanny in uniform you should have hired one, to avoid the embarrassment of the wrong clothes. Treat her like a daughter. Have a word, then give up! She's only there to help with the children. if she does that well, she's fulfilled her part of the bargain. You don't have to feel shamed by it all. She's not there to uphold the family image!