I've always liked the saying, "Blame the deed, not the breed."
In my personal experience, I'm a nanny for a family that has an English Mastiff and I've never seen a dog more devoted to and tolerant of children. The children are young (under two years) and Gunner allows them to crawl over him, pull up his lips and ears, tie things around his neck, and on and on. Their two lab females, on the other hand, I generally encourage the kids to leave them alone because I've witnessed both growl and be cranky with the kids.
I have a young English Mastiff (7 months) who also absolutely adores children. He actually shakes with excitement when he my niece and nephew come to visit. I was at the vet one day and he was, of course, overly excited with all the other dogs around. And then a kid in a stroller came in, and you would have thought he was going to have a stroke from the excitement. He spent the rest of our wait laying a couple of feet from the stroller with his head resting on his paws, tale wagging gently. He couldnt have cared less about the other dogs anymore.