A: You have the solution to your problem right in front of you. Let me ask you about these three dogs. Are they hungry? Have you fed them recently? How quickly could they develop a taste for human flesh?
Now, about the grandparents: Are they fit? Do they keep active? Could they outrun a dog? How about three?
These are the questions you should be asking yourself. Constantly. Out loud. In front of your grandparents. After a few days, they'll realize you're pretty fucking serious about getting a bunny.