You need to stop allowing this. It's completely unacceptable.
I know it can be hard to speak up, but the first time is the hardest - after that it will get easier.
Just ask to talk w/ your MB. Add up all of the hours you have worked (for whatever period of time this has been an issue) for which you have not been paid. Do the math of what that cost is. Then, if you want, you can say that you weren't wanting to press because of all the snow days etc... but that it isn't possible for you to continue working this amount of overtime without payment.
Then let her respond.
I'm an MB. I would never treat my nanny this way - it's completely unfair and illegal. If anything got to the point where she had to speak to me about it I would be mortified. I greatly value flexibility from our nanny when we need it, but I pay her appropriately, give her plenty of notice, and thank her profusely. Your employer should be ashamed of herself.
But you do need to address it. You, and the MB, are grown-ups. You are a professional, and you are entitled to payment for your time and services.
Pretend you're advocating for your best friend or sister if that helps give you some courage, but go stick up for yourself!
Good luck.