I believe I checked this out from our library in Kentucky somewhere between 1986 and 1995. I remember it as a short book, less than a hundred pages, maybe less than thirty. A pair of children (at least. There could have been more. And I don't remember if they were related to each other) decide that a lady in their neighborhood is a witch. When they confront her on it, they either convince her or she decides to play along. Either way, they convince her to try a flying ointment they got from a library book (it involved some sort of grease and soot). After an abortive attempt to launch herself from a roof, the "witch" proceeds to make short hops in the yard, broom astride, and claiming that she was jumping higher than she could before, implying that the ointment was doing something. I was recently reminded of the book when a friend was mentioning how some of the old "flying ointments" had psychoactive components, implying that the "flying" was a matter of skyclad witches standing astride dosed brooms and getting a contact high through their mucous membranes. Of course, this book, being a children's book, involved a fully clothed witch.
I believe the illustrations were done in a pen-and-ink manner. I remember there being one of the attempted flight.
I don't think that it was Jennifer, Hecate, Macbeth, William McKinley, and Me, Elizabeth despite a common theme of a flying ointment and witches, as I clearly remember the "witch" as an older female. Neither do I believe it to be The Active Enzyme, Lemon Freshened Junior High School Witch, which some reviews also note has a flying ointment, but also that it's all kids trying it out.