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" So it seems she is happy to work like a slave"
She is probably not happy at all. More likely so scared that she can not get another job as she has a family in her home country that she needs to look after. As a result of her stress, her work suffers, her mental health suffers, the children she is caring for may suffer... I am sure that the employer would know the rules about employee staff and they should. Some people don't see the connection between a content employee and a job well done.
I don't know the situation that was cited, but with our helper, she chooses to work on some of her days off. I do not work her like a slave. I told both our helpers that we appreciate their help (making it easier on us) and if they choose to work on their days off, we will pay them extra for their work. One chooses to take her days off to see friends, the other wants to send more money home, and we are fine with both decisions. What I am trying to say is that just becos a helper works on her day off, it does not automatically mean she is treated like a slave, sometimes it is the helper's personal choice.
As for curfew, I ask my helpers to come back before 10pm and they are fine with it, in fact, they come back mostly before 9pm. I was told that there was not much to do after 10pm. In any case, I think since clubbing or staying out very late was not generally something they did in their home country, they are not really asked to give up something they view as normality/necessity. On the occasions that my helpers had something special going on and needed extra time out, we were fine with it. We just wanted to know that they are safe and when to expect them home.
Obviously, if you are of the school who thinks that it is ok for helpers to hang around nightclubs, run around with lots of different men, drink and smoke and come back all hours during their day off (being their own biz and no one elses), then it's great, cos that means you will never have a single worry or fear about your helper ever.
If like me, you worry about your helper smoking, getting pregnant etc etc, I do not think it's terribly unreasonable to impose a few ground rules. I know having a curfew does not mean a helper would not get up to no good, but it does limit some less desirable places or acts that one associates with nighttime. I assume this is also the reason why we try to tell teenagers not to stay out so late. But I think these house rules should be laid out when one is hiring, so both sides can know what to expect of each other. If a helper thinks it's unreasonable, then she has the choice of not working for that particular employer.