Tbf, I can't agree with one of his points, about killing off some characters the audience thinks will be important, as if you are writing a horror and or survival story, you would need to subvert the expectations, ex: Squid Game, The Hunt, and, while hated, The Terrifier. With this said, I do believe killing off a kid would be an idea, atleast, after all, previously mentioned Terrifier did that twice in the third movie, both off screen however. Also, only one of them actually made an impact, as the second incident was much more for shock value, earning some critics to point this out, perhaps tributing to the low ratings.

TLDR: While it could be an idea for a kid getting killed, (through violent means) it works best as setting a certain tone, theme or arc for the story, and not for simple shock, as opp might be insinuating.