i would find myself very uncomfortable giving my au pair or nanny some money to buy their own food. If someone lives in my house to look after my children, she eats like them which is like us and if she wants a snack she helps herself to whatever (except what is needed for the next meal).
How embarrassing too to get nuice stuff for you and basic stuffs for her...
When the kids eat sea bass for lunch for example, the nanny eats sea bass too and with them, well that is the way I was raised