Fun Size (2012)

Wren (Victoria Justice) is a high-school senior who can't wait to get away from her dysfunctional family. On Halloween, Wren's mother decides to go out with her much-younger boyfriend, leaving Wren to look after little brother Albert. When Wren is distracted by an invitation to the party of the year, Albert disappears into a sea of trick-or-treaters. Enlisting the aid of her sassy friend, April (Jane Levy), and two other classmates, Wren sets out on a frantic search for her sibling.