My bet is that there was 'horeseplay' by the child involved - perhaps he kicked or pushed the mirror first ... This was the case with us. The mirror was on the floor leaning against the wall, and we were a little animated trying to amuse ourselves while the person we were with made his purchase.
However, it was a public place, and we weren't wild.
It doesn't matter whether there was horseplay or not. The child was in the store as an invitee. The store had a duty of care to make sure the kid wasn't hurt. The store failed. As the lawyers like to say:
Res ipsa loquitur. The thing speaks for itself.
To the child, peace; to the family, prayers and wishes for healing. To the store: get a very good lawyer. You'll need it.