Except he's already killed children. And the perception that he regretted it, he didn't sound too much like he did. He was crying, sure, but that was more over his mother and not the children.
"They're animals and I slaughtered them like animals" is not evidence of regret. It's evidence that he's dehumanizing his victims. He's a bad actor so it doesn't come across as well as it should.
Let's look at it:
He's already slaughtered women and children.
He's already started slaughtering Jedi by the time he reaches the children. So, we see an immediate foot-in-the-door phenomenon paired with his previous slaughter. We also see him surrounded by Stormtroopers, who can serve to provide the presence of a close authority figure that we see from Milgram's obedience experiments.
It's not a giant leap at all for him to slaughter children. Especially considering he's done it already by that point.