The kids case is one where camera has a clear benefit I'd say. Even if you have clear visibility to the rear window, you just can't see anything below the window height. And anyone coming from the sides you'd not see through the rear window either - but a very open vehicle might be a tad better through the side windows? Although with cameras being added to side mirrors or vehicle corners (and large screens!), that can also be expanded.

Total substitute. And they have to be, because once you have them you ONLY look at the screen. There is no chance I'm even glancing in the useless top mirror, nor over my shoulder. I do switch between rear camera and 360 camera though, but it varies between car brands if that's necessary. In the best implementations you see both at the same time.