In this case, discretion is the better part of valor.
It's unthinkable that a group of adults would attack a kid, but it's unthinkable that an adult would tear a sign away from a 3-year old kid and smirk at the crying child in the first place. Even if the rest of those people didn't attack the kid, she would probably would have been injured in the 10 to 1 assault on the dad that would likely follow his starting to fight the dude with the smirk.
I never suggested this. One, he could have said something
to them. If he did, I don't know what it was. Two, he could have made a complaint. Three, he could have had people with him who would stand up for them. It doesn't make sense to go into a crowd of these people as a solo protester.