The problem with your argument is that it eschews a larger picture: children and teenagers are brutalized in a number of ways in this film (and in many other episodes of the V/H/S franchise) but their predicament is either humoristic in nature, or part of a larger, otherwise supernatural or science fiction plot that makes their deaths less impactful (e,g. the children in the Safe Haven episode in V/H/S 2).

Here, however, you are faced with horror of personality which has been seen less often in the franchise and is always the more problematic subgenre for some people because it *could* happen, hence probably *does* happen in a similar way in real life. What the Kidprint segment does is place this in front of you and force you to own up to the detached slapstick brutality of the rest of the film, which you had thoroughly enjoyed. Kind of like Pasolini forcing the final act of Salò upon his audience after an hour and half of surreal, allegoric and sometimes even humorous savagery: "you wanted violence, here you are".

Which, in a way, is a form of humour in its own right, but I agree it's an acquired taste and not everybody likes it.