Back on dogs: He's a mastiff, not a child killer. Actually, if he's out in public and you are yelling out your own child, a child he doesn't even know, he will go on high alert to make sure your child doesn't get hurt. If anybody threatens your child in any way, even though he's never met your child before, and just because he knows that this little person is a child, he will go into all out protective mode. Every child in his range is fully protected from kidnapping or worse, as he'd do everything in his power to take down the person hurting any child, no matter if said child is one of his boys or just some random stranger.
That said, I ran into another "Giant dog must be horrible!" person a few days ago. If you're seriously that afraid of big dogs, don't go to pet stores where they're welcome.