It was supposedly in a cage and the boy put his finger in. What do you expect? The dog was probably aggravated and chomped down and because of the kids' small size, he bit the finger off.
I don't know how to feel about killing the dog and cutting it's stomach open to retrieve the finger. It's very unsanitary, the finger was probably also subject to digestion, and unlike clear severs of fingers by say a knife, the dog probably severed it too severely to reattach.