If the father knew, though, why didn’t he stop him?
It's possible that the perp had taken one of those horrible drugs that gives superhuman strength. Cops have had to taze people repeatedly to subdue them when they are under the influence. Also, the article pointed out that the perp used the one child as a human shield, threatening to strangle it, when the cops did arrive. So his own older father with no training did the only thing he knew to do -- call in the experienced officers.