So, it was all really the doing of...(spoilers)
The Boogeyman?!? I mean the kids seemed to have spelled his name wrong but they're kids--I'm sure he's not a disco-dancing serial killer.
Anyway I dunno if that's all that believable; and, in fact, I think this movie unfairly characterises The Boogeyman as someone (something?) who would do such a thing. I don't think there's ever been a case of the Boogeyman killing families. I think he just likes to hide in kids' closets and under their beds and stuff to scare them at night.
It's more plausible these kids are the killers themselves and just blamed the Boogeyman for their issues. Then again that wouldn't explain how all the killings are similar, connected, and how the kids disappear into Boogeyman-land afterwards.
At one point in the movie I actually thought the over-eager police officer was that one kid, all grown up... That it was each kid growing up and passing on to a new kid, the practice of killing their own family and then teaching the next kid. True twisted serial killing style. But nope...
The movie was not that scary TBH, but that was especially the case once they exposed the killer in the first kid's drawing--depicting the alleged "Mr. Boogie". At that point I laughed that they would blame this all on our lovable (loathe-able?) childhood friend. But definitely don't let kids watch this because if they're scared of The Boogeyman already, your comforting them in telling them "he doesn't actually hurt anyone" might not be believed, them preferring to believe the fairy tale shown here instead.
6/10? Nah, let's go with 5/10.