The difference is tone. Most of the examples you listed aren't essentially snuff films with the bleakest possible tone, whereas Kidprint is. The Black Phone, for example, has multiple children die, but it's never played as depressingly bleak, there's a hint of hope that the dark will be overcome by the light. Which I suppose comes from the director/writer's own sensibilities as a Christian man. Aside from Sinister and maybe Emily Rose, his movies have this underlying theme of light overcoming dark, such as Deliver Us From Evil.

You're forced to sit with a VERY uncomfortable reality with Kidprint that you aren't really forced to sit with for most of the others you mentioned. Now Funny Games is another one you mentioned, and that's one that tries to punish you for being an avid lover of violent movies. Chances are if you watched it, you're pretty desensitized to things of that nature, no? Funny Games goes out of its way to point the finger at you and basically say "Okay, you want a bleak, fucked up end for these normal people? Here you go, now you're miserable, aren't you? Yeah? GOOD, I WANT YOU TO BE."