Hi all --
So my tuppence is that you should pay a "fair market rate". And in my view, the best way to work out what the fair market rate is for the area you live is to ask as many people as possible in your area what they are paying. That way, when they meet other au pairs in the park, they will be roughly on the same rate and there will be no resentment - and she's much less likely to get poached.
To get you started I asked 7 of my Mum friends (all living between the commons) who have live in Nannies and au pairs what they are paying. Here are the results for you:-
3 live-in nannies: between £300-350 per week net
These are professional experienced nannies working approx 45-60 hours a week plus 2 nights babysitting, one during the week and one at the weekend. They get room and bills included but not food, toiletries etc. Mostly, they do not eat with the family or "hang out" with them unless specifically invited to do so.
4 au pairs: between £80-£110 per week net (shock horror everybody!
)
These typcially "work" 20-25 hours per week and are as @HM says viewed like "big sisters" (or brothers). They get their board and lodging, food, toiletries, phone etc paid for and typically will eat with the family, as if they were a member of the family. The rate does not typically include babysitting so they are encouraged to find/do babysitting, or sometimes cleaning,on the side which can help them supplement their "pocket money".
Just as a footnote, I'm just coming off maternity leave and during that time I paid myself £400 (pocket money) pcm to see how far that would go. It was more than enough (excluding all the things an au pair would get like food, toiletries, household bills etc.) But then I included wine in the "household bills" and I don't have an expensive Amazon habit (yet)
Hope it works out @HM
xx