User blog comment:Tinknme96/Kid's costumes/@comment-26818419-20161024220202

The Trick-or-Treat costumes are exclusive to the Treehouse of Horror Events.

Unlocking them this year (2016) means being able to send the kids on their Trick-or-Treat jobs, in order to get more currency (Obols, Clapboards, etc). This is different from previous Treehouse of Horror Events, where they were all automatically "unlocked" near the start of the event.

Regardless, every year the jobs, and therefore the halloween costumes, disappear. There are certain costumes like Clockwork Bart and Saxophone Lisa that the kids don't trick-or-treat in but you keep permanently, but that's it.