Sorry, Op, but you're wrong. You aren't "basically working late for no additional pay - which is usually the incentive to stay late." You are working the same hours, just scheduled differently. Personally, I would not pay your more for the situation you describe. Not that that matters, because I wouldn't randomly change a nanny's schedule.
It's fine to be annoyed at the schedule change. I would be, too. But if the MB decides she needs coverage for a different set of hours, the only recourse you really have is to find a new job with hours that better match what you want. I don't think you can expect her to pay more for a schedule shift. That just isn't reasonable.
Of course, if you talk to her and tell her the schedule change is a deal killer for you and you will leave, she may revert to your old schedule. The risk in that is that she might also fire you before you have a chance to secure a new position. Which would be tough because ideal PT jobs are hard to come by. GL.