The concept is pretty obvious, and I'm guessing a number of such businesses exist in smaller form. Maybe there is one larger scale.
Scale is not important, what is important is widespread.
The basis is seeing what has happened to children left to be babysat, by even the nicest and friendliest of people, after the parents are out of sight. Hidden nanny cams have proven that even the nice teenage neighbor girl can be a monster when left alone with a child.
So for a small fee, hopefully paid by parents/government/charities, the teenage girl next door can become a member of the plan. This requires a short briefing, or can be longer training as appropriate.
The basic rule is the babysitter always has the children in view on camera, and themselves. Whatever necessary to assure nothing is going wrong. The cameras are provided on a loan basis, is my initial thought.
The service then also provides that if the babysitter needs help, that someone monitoring can jump in with advice and/or dispatching whoever is needed to the location.
Recordings made are kept very discreet and are only available on a limited basis as needed.