Thats horrible, I do sympathise, my own gsd dog attacked my sheep (out of character and unprovoked obviously) after spending years happily side by side with them, we had her pts, as she seemed to have snapped. (she also went for a person the same week).
trouble is once they've done it once they re-offend, unless you have fantastic fencing, it could well come back.
The dog I have now wants to chase everything, pigs sheep, chickens, ducks. We have run a tape of electric fencing about 20" high all around all our fields, dog touched it once, never again! its the only thing thats worked for us.
As for the piglet, they can heal incredibly well, purple spray is amazing, but just keep an eye they dont get infected in which case get them anti-biotics asap.
Legally you could have shot the dog, maybe next time eh!