nanny overtime

I have a nanny who is scheduled to work 40 hours a week, with 1 wk sick, 1 wk vaca and paid holidays to start. Our family takes more than 1 week vacation every year, but cannot afford to pay her for time that she won't work while we are away. (Approx 4 weeks a year.)

I would like to give her the opportunity to make up for the unpaid leave time by working extra hours when she can/wants to), but i am worried that this will then require me to pay her overtime for choosing to work those additional hours.

For instance. I am planning on taking 2 days off at the end of february and would like to allow her to work an extra hour on whatever days she wants during the entire month to make up for the days she won't be here. This would mean she would work 42-43 hours some weeks.

She is not required to do this and she took the job knowing that the time was unpaid.

Thanks!