I believe my kids babysitter may be ignoring them, any suggestions how I can record the babysitter?

I have a feeling that the babysitter where I leave my 2 year old maybe just ignoring the kids for part of the day.

Any suggestion where I can sneak a recoding device (voice or video) onto my kid/ her possessions, so I could find out?

The baby sitter is not in my house so I cant use one of those nanny cam dolls.

Thanks