I know a family with 3 children. They don't have au pairs any more. They would sometimes leave their children in full charge of an au pair from 7am - 8pm. Two of the children were at school but the third was only 3 at the time though she was in part time nursery 3 days a week.
The difference? They had TWO au pairs who shared the work between them and during term time at least they we free except for an hour or two of housework from 8-5 4 days a week. They had one day a week each of looking after the little one. Also anything they did over 33 hours a week was paid at NMW - i.e. baby sitting, covering for other au pair if they were sick. They were also expected to help the children learn Spanish (which since they were both Spanish probably wasn't a major problem)
They were paid £110 a week plus bus pass, all meals and snacks and allowed to phone home as often as they liked.
Not perfect but I know which job I'd rather have