Since you only said Mastiffs, I guess that would include Cody, my Tibetan Mastiff. TMs are not as large as other mastiff breeds, but in my experience are great with children. They were originally used to guard villages of women and children. We got Cody when he was 2 and he had never been around many people and certainly not children. He had spent his entire life in a kennel, with no company other than the other dogs there. He is great with my grandchildren, 8 and 2. When we took him to the craft fair, strangers children would hug and love on him and he patiently tolerated everything they did. When he laid down on the sidewalk, while we were talking, 2 small children, maybe 2 and 4, laid down on him and cuddled up with him. He just laid there with his tongue hanging out, like he was smiling. He doesn't lick or lean on people and keeps his tail over his back, not wagging it back and forth, so that is not an issue. He is not mouthy at all and doesn't get excited or jump around, ever. He is very guardy at home and growls, barks at strangers coming in the house, but I have never seen this with a child. When I babysit, I put the boys in my sitting room to watch TV. Cody stays in the kitchen with me and if they walk out of the bedroom, he growls, really low, doesn't even pick up his head, just lets me know they are coming out of the bedroom. I don't know about all TMs, but he is great with children and small animals and just wants to protect them from other dogs.
Kathy