Well, that would certainly make sence if they had planned a big orgy in another apartment. If, If, BIG IF, this where the case, than an accidental OD, or other accident would best explain Madeline's death. If another child was vomiting, then maybe they OD more than one child. Maddie could have aspirated her own vomit. All the children could have been affected in some way shape or form.
I just can't see that many people keeping quite, unless they ALL did something sinister that night...and drugging their children for group sex would put them all behind bars, and strip their licence to practice medicine.
do you think the mother who was there was also in on the orgy ? But strange to take your mum on a sex/swinging holiday - I never did