OP - I assume this thread has trended, hence the unhelpful responses from the ignorant masses.
With our first au pair who lived in our house, I just put a black star in marker pen on anything that was for a dinner, so that it didn't get eaten.
Subsequent au pairs had their own flat next door, however, as the au pair regs say that they have to live as part of the family, we insist that they eat with us at least once a week - usually roast on a Sunday.
We now give them a Sainsbury's card with money on it to buy their own food and we have an account at the local butcher and deli. I tell them the spend is about £35/week at Sainsbury's, but if they want to spend more they can. I wouldn't want anyone to be hungry.
We've had 7 au pairs now - their spend has ranged from £10/week (I was slightly worried about that one) to our French au pair, who was my butcher's favourite ever customer as she could consume vast amounts of meat.
I also think with au pairs you need to assume that they know nothing. For example when you asked her to supervise them, tell her exactly what she needs to go "Please would you go outside and supervise the children, they need watching quite closely so that they don't; hurt themselves."
You need to also tell her that phone use needs to be kept to an absolute minimum during her hours.