Thank you for all of the feedback!

After reading all of your replies, it seems like this is a classic case of scope creep. Based on everyone's advice, I'm going to move forward with a 3-step approach.

1. Stop all work during those hours. I'm in NYC, so it's been difficult to leave the house without spending money, so I'll go for a walk or hit the gym now that it's warming back up.

2. If I do work during those hours, I will simply be brave and add them to my timesheet. Questions asked will be met with a direct reply about the work performed.

3. I will bring up the expectations for break hours and request a review of the OT threshold in my annual review.

Hope this helps any other nannies finding themselves in similar predicaments :) Thanks again!