"Nanny cam" video is legal in all 50 states. Most if not all have rules that dont permit recording in "private areas" like bathrooms and bedrooms.

Audio recording without consent is legal in 2/3 of the states. The other 1/3 requires that you notify/receive consent. The best way to do this IMO is in writing in an employee agreement of some sort.

If it were me...i would just do it anyway. The primary goal is to keep the kids safe. If you ever have an issue just strip the audio from the video and present that as evidence.