What do you call banging a girl with Down's syndrome on Halloween? A monster mash
What are you trying to say about children with Down's Syndrome?
I hope the girl with Down's Syndrome is ugly and promiscuous.
Calling people born with Down's syndrome monsters is a terribly offensive stereotype, that would really hurt the people who love them and care for them.  And to suggest, even in jest, that you make a sport out of having sex with people who are underage and unable to consent suggests actual illegal behavior, so I would advise even against joking that way.