How do you know the kid was playing? If the mirror was properly secured, then how could a 3 year old have been strong enough to cause it to fall? Especially if it took two adults to lift it off his body. Presumably they were in the clothing section when this happened, so this wasn't a dangerous area. A person should be able to turn their head for a few moments without worrying about a heavy item falling. If an old lady's scooter nudged into the mirror and it fell on her, would wouldn't that be equally tragic and senseless?
Walmart--any store--should leave ample room for customers to shop, aisles free of clutter and spills and heavy items secured safely. That IS their responsibility.
And I agree about having ample room and clutter free ailses. That's one reason why I typically avoid my local Wal Mart.