Trick-or-Treat, Smell My Feet! by Lisa Desimini. 2005. Hardcover (Ch-PIC)

After secretly gathering stinky socks from the neighborhood, two witches play a smelly Halloween prank on trick-or-treaters.

Delia and Ophelia are witches. They're cooking up a particularly naughty spell for Halloween, and the main ingredient is super-stinky socks. That night, when the neighborhood children go trick-or-treating, the only words they can say are "Smell my feet!" Of course, no one gives them any candy, much to the delight of the two witches. The fun comes to an end when a mysterious ingredient causes the potion to backfire--returning the children to normal, and putting the witches in a very strange situation!