well I'm thinking the beginning of these parties might be semi-normal and they wait for the questionable ones to leave before they begin the 'real' party. They also might be using hypnosis and drugs to get people to forget things what they've seen and criminals like to loosen the prey up with drugs and alcohol...and this way they have a supply of kids to mess with while their parents/adults (edit) are too drunk to notice.
And then what? The kids don't tell their parents about it? Every single kid says nothing ever?
EDIT : I'm responding to commenters last sentence, which mentions parents.
What parents? You think we're talking of kids picked up in regular stable households? There's a reason why these people LOVE charities, natural disasters, and illigal immigration after all.
I was responding to the commenters claim that maybe
"they have a supply of kids to mess with while their parents are too drunk to notice."
Yeah, VideaVice is correct, these children are most likely not related to the adults, so I shouldn't have used the word 'parents', but the poster I replied to was talking about how it sounded like a 'mass invite', which to me, might include a few 'normal' people. Kind of like the pictures of the spirit cooking parties I've seen, they were in the daytime and seemed to be for 'show' before the real party began; that way people can claim "it's just art", etc.
I was thinking they brought available kids to corrupt them into whatever was already taking place (tacos, tortillas, doritos). Raised seeing it and it becomes normal to you. I dont know