Our nanny started at 25-30 hours, now more like 35-40, but all along we have offered paid federal holidays if they fall on a day she'd be working, 2 weeks vacation (at a number of hours consistent with what she works--i.e. it was 25 hours, then 30, now 36), and 1 week of sick time/PTO.
We also offer guaranteed hours in the sense that if we don't need her or she leaves early due to our travel plans or a visiting grandparent we pay her for a full day.