A good guess, but I think this is unlikely to be a scene from Halloween. Generally, during Halloween, people behind the doors *give* candy to the children who come to the door, rather than *buy* candy from the children.
I think this is a scenario where girl scouts come to the door to sell cookies to fundraise. In America, girl (and boy) scouts often sell cookies to finance their operations, and "buttercup cookies" are a popular type of cookies.
The girls are saying "buttercup girls" basically to mean "Hi, we're girl scouts and we're selling buttercup cookies!"
PS Americans do not trick-or-treat during Christmas, but during Halloween! :)