Karen's Witch (Babysitter's Club series), by Ann M. Martin

Karen's Witch is about a little girl that thinks her neighbor is a witch, because her neighbor (Morbidda Destiny or Mrs.Porter) plants herbs and wears long black robes. Karen is young and her parents are divorced and Mrs.Porter lives next door to her dad's house and she loves going there and she just talks about witches and all the other magical things but no one believes her. So one night she sees a witch come out of her house on a broom! So she tells her friend and she comes over. They are spying on Mrs. Porter and they hear her say to her black cat "twelve o' clock" so the next day Karen sees lots of people come over and she runs over to her friends house and they go to Mrs. Porter's house quietly and shout "WERE COMING TO GET ALL YOU WITCHES OUT OF HERE!!!" And guess what happened next?

What I just love about this book is that it scares you just a little bit because you don't know if Karen's going to get in trouble or if Mrs. Porter is actually a Witch!