[Link: mwl-law.com]

In short, even with One Party consent states. The recording of audio to keep the nanny in check would be illegal. Unless they are the party that knows about it. But seeing how it's the home owner who wants it installed, I can assume the nanny will not know about it.

So you, as the installer/business owner, may want to ask a local lawyer what to do. Meaning if you should have the lawyer draft up a letter stating that you will not get in trouble in any way, shape or form if shit hits the fan. As you were just the installer.

KOT