I can tell you what mine is supposed to do...
Average of 3 hours housework and 3 hours childcare per day. Housework is light cleaning and tidying every room except ours, and washing/ironing for the children. Childcare is taking children to and from school and looking after children when I am out.
2 evenings of babysitting per week.
My au pair's hours are 8-11am and 3-6pm, or 8-9am and 1-6pm on language class days. If we are going out and I need the children fed before 6pm, I can ask her to do it.
I wouldn't ask her to prepare food outside of her normal work hours. Something that we don't get our au pair to do, but is a reasonable duty, is running to the shop for a pint of milk and loaf of bread.
My au pair doesn't do many of her duties very well, so we are letting her go at Christmas. She's our first au pair, and I can see how we made mistakes about communicating expectations - it's very hard to tighten up once you've become accustomed to a certain standard (especially with the language barriers). With our next au pair, we will not make the same mistakes.
With lots of small children, I think it would also be reasonable if you got help with the bed/bath routine, but obviously the work hours would have to be right for this.
I don't think you can really take advantage of an au pair unless you are making them work longer hours, making them do heavy work, leaving them for a long time in charge of young children, or not letting them go to language classes. I don't see that it makes a big difference as to their actual chores, as long as it doesn't run into their free-time.