IMO parents should protect and guard their children (and leave the rest to Allah SWT) not a dog. No matter how skilled or trained… in the end a dog is nothing more then just an animal. Besides as a Muslim it is too much work to keep a dog (ie having to keep away from it’s saliva, not letting it into the home etc). Nowadays, they have security alarms and all kinds of technology to guard and protect your family and home…there really isnt much much need for a watch dog.
To answer your query…
-from my understanding it is not the dog that is impure but the saliva from a dog is what is considered to be filthy.
http://qa.sunnipath.com/issue_view.asp?HD=1&ID=1385&CATE=3
I am not sure about the rulings of keeping the dog outside of the house and therefore can not answer.