This is a dangerous and - quite likely - illegal breed of dog. While I would not normally wish to see any healthy animal put to sleep, what happened to your poor cat could very easily happen to a child sooner or later and so you have a duty to report what has happened to the police. Make sure you let them know that the dog is out of control and tell them what has already happened and what you fear may happen in the future. The police will investigate and, if what you've said is true, will remove the dog from their ownership prior to it being humanely euthanised. Secondly, since the dog's owners are clearly incapable of controlling their animal, they may well be legally banned from keeping a dog for a certain period of time.
Various dog lovers are going to attack me for this answer. My only answer is that I have been a strict vegan for more than ten years and regularly work with animal rights organisations, and as I said at the beginning I would not normally want to see any healthy animal put down. However, it doesn't matter how much you like dogs when the lives of children are endangered.