as burnhal says it depends on the events beforehand, the dog is old, and the child could have pestered her, or even startled the dog while it was asleep.
I had a BC when I first got married, and although she had been fine with my son for 12 months, when he was walking around he ran up to her and hit her on the head with one of his toys when she was sleeping. Now I know I should have stopped him, but it all happened so quickly....her reaction was to snap and he had to have three stitches to three small puncture wounds in his cheek. We didnt have her put to sleep and my mother had her, she was never left unsupervised with him, but didnt go up and attack him for no reason.....
There is no reason why the dog would do it again, but I would obviously keep the dog in another room when children visit.