I agree the snowboarder was at fault. As for why they were stopped in the middle of the run, no one has considered the likelihood that the kid had fallen and had gotten up? Maybe he didn't fall, but it's possible he literally came to unintentional stop as a result of a heavy wedge turn. I see kids all day who are "continuously skiing" that frequently stop or nearly stop as their turning halts their modest momentum.
One thing though. Looking uphill before starting does not only apply to trail mergers. If I stop on a run, to the side, or smack in the middle, it is my responsibility to look uphill before I continue to ski.
I think the real issue is that the snowboarder had time to recognize the situation developing. If you watch closely, even if the small kid he hit wasn't there, he would've run over the back of the guys skis or at least been ridiculously close to colliding with him. Yes, that little girl made an unpredictable hard right and pinched the rider, but he could made a hard right turn himself and went around the dad, or just hit the brakes and stopped. He had been riding alongside that girl for the majority of that run. Anyone knows who skis that if you stay close to a young inexperienced skier, you're basically putting yourself in a bad situation. He should've passed her sooner, or fell back and given space. It's like driving alongside a drunk driver for a a couple of miles. You've watched him swerve all over and make questionable choices, so why the fuck would you continue to drive alongside him?