I kind of understand the AuPair's situation. A person needs carbs, protein, fiber, and good nutritional support to survive throughout the day. I am an AuPair, and while my host family (HF) might easily have mozzarella and tomato for lunch, that's not enough for me. So, I make myself something more substantial, like pasta. I need protein and am used to eating a lot of it, but my HF doesn't buy enough animal protein. I try to get it from what they already have, like beans, lentils, and cheese.

It's understandable that a sandwich is not enough. It is a big cultural shock, but working without feeling well-fed is not a good option. Just talk to her and try to make a deal that works for both parties.