Not having the audio to add to our awareness, it really appeared something about the woman’s change in behavior when walking away after injuring the first child alerted the parents and they started pulling away. Almost as if there was a verbal threat the wife heard as she started her second approach. And she was fixated on the first child at that time. Maybe the wife saw the blade?
And remember, even a moderately sharp knife can cause an easy laceration with little pain at first to the injured. The child may not have really known what was going on until he/she saw blood on their hand. The attack almost looked like a caress, with a razor blade, straight razor, or anything 2 inches or less would have been easy conceal, easy to lacerate.
@Jerzy If I had known what happened the first time, and the woman turned to come back again - as she did… I’d have been right there with you. An imminent threat of great bodily harm, opportunity, and apparent intent. The totality of things in that moment would have caused me to draw my weapon find a clear angle to backdrop and stop the threat.
Afterthought: I’d try and detain her too. She’s already proven to be a threat to others, it would be unknown how soon after she left the immediate surrounding that she could attack again. I’d be screaming my head off warning others near her if she didn’t respond to my efforts to keep her there until authorities arrived / medical care came 0r we bugged out to nearest urgent care site.