This wouldn't be any different from putting hidden cameras in your house when the babysitter is over. You're not in a public place, so you should have a reasonable expectation of privacy.
Nope, wrong. It's your house. You can put all the cameras you want inside of it. There are no restrictions..
So to take that argument to its conclusion, then it is OK to place a hidden camera in a bathroom where babysitter might be bathing, changing, or other state of partial undress?