The Five Masks of Dr. Screem by R.L. Stine

It's Halloween and Monica Anderson is out Trick-or-Treating with her younger brother. They knock on the door of an old, creepy house and are met by a strange woman who pleads for their help. Every year she must battle for the control of five sacred masks with the evil Dr. Screem. These artifacts have the power to manipulate the world around them. Monica doesn't want to help at first but then Screem bursts in and steals the masks. This sets her and her brother off on a Halloween mission that will change their lives forever