This is getting a bit silly. I only put it forward as what I might do (hypothetiacally) if I really wanted to ensure someone stayed longer. It was in reponse the the contractual clause notes in the OP, whiuch even if laeful, in my view, is unreasonable.
I have it on good authority (form past and current nannies au pairs) that I am fab employer. So I don't really need advice on how to be nice to them. genrally speaking.
But I do think that nannies are employees, and if the seek to be seen as the professionals that they are, than their employment contracts should be governed by the same rules as any other. And I'm sure having a six month notice period is not unheard of.
I did write a 12 month fixed term contract with a previous au pair. After she arrived, got to us and we got know her a bit a couple of weeks I said I didn't really really fancy replacing her and she didn't really fancy being let go. So, together we dicided to lock the employment term down to 12 month contract in which neither of us had the right to terminate the arrangement. Now, of course, if she had had some personal tragedy or something I never would have held her to it.
But, let's face it, nannies sometime up and leave cause Johnny down the street offered the job she really wanted three months after starting yours. And that is what parents want to try and minimise. Of course, it happens anyway sometimes. And I for one could never be bothered to go through the process MrA describes. But, I might try and write a contract that would reduce the chances of finding myself in that position.