Here's my take: For the entire duration of my childhood, I was susrrounded by no less than 5 Siberian Huskies and/or St. Bernards. The only time I got bit was when I was a stupid kid and tried to break up a dog fight by sticking my hand in the middle of it, not because the dog turned on me.
When it comes down to it, you know your dog and if it was bred resposibly and is being socialized and raised correctly. You shouldn't have any problems.
Unfortunately, you are not going to change your grandfather's mind. Sounds like he is set in his ways. My father in law is the same way...He dislikes Sierra simply because he thinks a Newfoundland is too big for our home. We live in a 3 bedroom bi-level! Anyway, back to your problem...You may want to try putting your foot down with him and let him know that you respect his opinion and his concern for your children, but this is a subject that you will not discuss with him and let him know that if he insists on talking about it, you will politely end your call because you don't want to fight with him. He will start getting the picture after the first couple times.
When it comes down to it, it is YOUR dog, YOUR home, YOUR children and YOUR decision and much to his disappointment, there is nothing he can do about it.