Your characterization of the service provided is "adult physically present in the vicinity of children to take care of issues". That sounds to me like a lower quality "service" than what e.g. my wife provides, which is actually raising them, teaching them, giving them emotional support, taking them on errands around town, etc. Even with your own kids it's way more difficult to give them as much attention when there's 1 vs 2, so I find the assertion that quality of care doesn't drop after 1 to be dubious as well.