Let's face it, being an au-pair is shit - it can sometimes descend into being servitude by another name. I really depends on the AP and their background in relation to the employer. The more the AP needs the position, the more the employer/host can influence the duties undertaken.
From wikipedia:
"The concept of the au pair originated in Europe after World War II. Before the war, an abundant supply of domestic servants had been available to look after the children of middle and upper-class families, but changes in social attitudes, and increases in wages and taxes after the war, made the old system inaccessible to most middle class parents. At the same time, social change increased the number of middle class girls who needed to earn their own living, and rising educational aspirations for girls made experiencing foreign cultures and learning foreign languages more common aspirations.
However, due to the stigma attached to being a "servant", an essentially working class status which even working-class people were repudiating, this potential supply for domestic labour could only be utilised if a new non-servant role was created. Thus the au pair was born. The au pair was supposed to be treated as a member of the family rather than a servant, and was not required to wear a uniform.
An au pair receives an allowance, and a private room. The usual practice is that au pairs eat with the family most of the time, and join in some of the usual family activities such as outings and trips. However, host families normally expect to have some private time to themselves, particularly in the evenings. During this time, an au pair might retire to his or her room to watch television, study, or go out with friends. Provision is often made for the au pair to have time for studying, especially of the language of the host country. The Council of Europe recommends that au pairs be issued standard contracts with their family."
When I was one the expected hours/responsibilities fluctuated almost daily. The employer doesn't realise that just because they are at home, doesn't mean the AP is - the AP is always at work.
So cut them some slack.