I'm a MB. The more you do for free, the more she will come to expect it and take it for granted. If you have not said anything, she probably thinks that you're ok with this, especially if when she says 'thank you' you just say 'no problem' etc.
If you need this job and don't want to rock the boat too much, you could just forget about what has happened so far in terms of payment but try to establish that you need to be paid going forward, e.g. "I really appreciate you including me in your wedding plans and was happy to help you out with looking after lovely little X & Y at the time and that you got to have a nice honeymoon. I am happy to continue to help you out while you are going out of town, but I just want to let you know that in the past whenever I have worked more than my regular hours I get paid some kind of overtime.' I think that is the absolute minimum you should say. If she is respectful and friendly, then you ought to be able to say more, i.e. be more clear about needing to be paid for your time, and have her understand. I would imagine that if you are having the kids to your house you are even paying for their food etc out of your own pocket, so you should be getting expenses of some kind too.
To be honest, though, she doesn't truly sound respectful ... maybe just manipulative?