English, where do you get the view that there's no evidence that he was going to touch Henry?
In the interests of research, I've forced myself to listen to That Scene again. The sequence was:
Rob shouted at Henry to go upstairs, he didn't;
Henry asked why Mummy was upset;
Rob said "I'm warning you, do as you're told this second or I'll .."
Henry: "No"
Rob: (growls): Right, come here you little ..
Helen: "No! Get your hands off him, (screams) don't touch him!"
(Various grunts and gasps)
Helen: "Oh, oh, sorry"
Rob: "Helen"
Helen: "Sorry"
Rob: "Put the knife down. Helen!"
Helen screams, more grunts and cries, sound of knife falling.
Sounds to me as if he was definitely going for Henry.