I'm with the judge in asking why the dog's owner, the little girl's uncle, wasn't also prosecuted for manslaughter.
As far as I understand the grandmother wasn't responsible for the dog. I take it that it wasn't her who was walking the dog off the lead in the street or taunting it as I heard was shown in one video.
The grandmother being prosecuted gives the green light for people who own dangerous dogs to take no responsibility for them and blame whoever they left them with if something does happen.