I would view this as any other job out there. If you work over time you get paid for it, but if a holiday falls on a day that you usually work over time, you would only get paid for 8 hours.

We had a nanny (Great Falls, VA) and paid her $20/hour for a 40 hour week. If she happened to work over the usual amount of hours a couple of days, we would pay her the over time. She also had all federal holidays off paid as a regular 8 hour day with no over time. She had two weeks vacation in which we gave her a week in the summer (which is when we went on vacation) and the week between Christmas and New Year's. She worked for us for 20 months and never complained about pay or anything for that matter.

I would say if it is not written in the contract about paying over time for holidays, then don't pay it. If she wanted a set salary that should have been negotiated as a salary not hourly. I would say it all depends on the feeling you have from her. If she is great with everyone, most importantly the kids, then why squabble over a few hours of over time. But, if you find that this may be the start of something that will quickly go down hill, I would start searching for another nanny.

While I agree he shouldn't try to renegotiate terms, I think the families are in the wrong.

When I've had guaranteed hours/pay it was the same for every week.

Is your nanny free to turn down the OT hours any week she chooses? If they're unnecessary then I could see your point. But if you expect her to keep those hours available for you then you should pay her for them when she's not needed. Otherwise she should take a job that will pay her those hours since that is far more common.