The Haunting Hour: The Bad-Babysitter by R.L. Stine

The Bad-Babysitter is a short story from the book The Haunting Hour. Matthew and Courtney are babysat by a babysitter named Lulu, but they didn't want to be babysat. They told Lulu about their annoying friends, so Lulu helped them make cookies out of mud in the shape of their friends. Then Matthew dropped one of the cookies and the arm fell off. The next day, the annoying friend had a broken arm. Then they realized they made voodoo dolls. They made a cookie of Lulu to scare her and the head fell off and Lulu never came back.