I want to catch up and see some of these other videos, but all I watched was the first rotty one, and WOW. Yes, I instantly saw the shifts in body language. The kid was really the one doing the most social harm on Team Mastiff, because he was being friendly and open to this butt muncher (please excuse me, but this guy is a complete freaking idiot). The two adults had closed body language, weren't engaging, and I think the man knew something was awry because he began stroking the mastiff when he started tensing, but he may not have been fully conscious of what was happening.
If I saw two rotts walking toward me like that and my dog was DA, I would either leave the area immediately or warn the guy from afar not to come within X feet of my family. If he parked himself where he did, frankly I would probably take out a nightstick. I would not trust that guy for a second to control those barking dogs, and that is a STROLLER. NO WAY would I be out with a baby and my dog and not have a weapon stashed somewhere. I'm a lot less friendly than most people, though, and I am very distrustful of arrogant, cocky, imposing people like this guy.
Bottom line: if one of those dogs got loose (and I've seen powerful breeds drag down their owners and break free quite violently), I would want to be prepared.
One day this guy is going to cause a serious fight, if this is how his "training" works, and I hope those dogs are rehomed.