He should have never laid a hand on her in front of the kids, regardless of whether you think he should have hit her or not. In my opinion, he should have just restrained her from hitting him, and said very calmly: "Calm down. Not in front of the children." Especially as this was being videotaped (clearly he knew it was).
Now, Mommy has ammunition to call Daddy a piece of shit for a very long time. This will only serve to poison their view of him.
I dated a girl who had severe depression, and once for the simple reason of me asking very politely "When is dinner going to be ready?", a fucking pan went flying right by my head. If that hot pan or its contents had hit me, I would have been severely injured.
Obviously, I was filled with rage and actually saw white for a few seconds and had tunnel vision. But, I let cooler heads prevail, took her apartment key off my ring, put it on the coffee table, got my jacket and left for good.