Dogs also spend a lot of time on leashes where they'll get yanked around if they don't pay attention to their human's movements. And many families specifically train and reward dogs to move in sync with them (heeling, agility). It seems obvious that synchrony is something that humans both train and selectively breed dogs for. I guess the point of this study is that synchrony is another way to measure social connection between dogs and humans and that this connection extends to children.