I obviously don't have all of the facts either, but usually -- though not always -- there is some fault on both sides.
A lot of assumed facts here, true. But if an object is placed in such a way that it could do harm to anyone--child, shopper, employee--then the ownership of the store is responsible for those who made that possible. Unless the little boy had a good set of tools and knew how to use them or was swinging an 8 pound sledge, someone screwed up. Just as easily could have fallen when a sweet old lady in a motorized car hit it or any number of other events that could be reasonably expected to happen over the course of the life of the store. Something like a five year old boy probably doing what five year old boys often do is one of those things. Either the mirror mount was poorly designed , or it was not installed as designed or had been damged at some point and not repaired. Those who would blame it on the parent are just silly, which is about the tamest description I could muster.