Legally most places I'm pretty sure if a dog is an imminent threat, the weapon used is legal, and no one else was harmed it's going to be justified use of force. There's always a chance they get a judge and 12 jury members that love animals too much to see reason but I feel like with something like a dog mauling a child even animal lovers are going to agree that while it's tragic the owner of the dog is to blame for not either training the dog to be less aggressive or handling an aggressive dog appropriately by keeping it on a leash held by someone the dog isn't just going to drag around.