Au Pairs don't usually get minimum wage because they are not supposed to work a full working week, usually live in with full room and board. If not, are they an au pair or a nanny/child minder, in which case of course they should have at least minimum wage.
I've had au pairs, each one stayed with us for a year. Had fantastic experiences, but each of ours were only expected to mind the children a maximum of four days a week, after school only, so probably about a 15/16 hour week. They lived with us, were treated like one of the family, were basically like a big sister to our children. We are still in touch with all our au pairs, years later, are in touch with their families and would highly recommend it to others- providing you provide them with the right support and circumstances. All of ours came to learn english for a year, and every single one wanted to stay more than a year!
However I have seen the other side of the situation. I know one family who went to work and had a phone call from the gardai who found their children wandering in the street...the new au pair was in her bedroom on the phone to her boyfriend with door closed. And I know another family who took on an au pair, and a day after she arrived, left her with their three children, only two of which were school going, and went skiing for a week.