Like many kids, cartoonist Ellen Forney grew up in fear of the demon that lived in the toilet. Each late-night trip to the bathroom was punctuated with the same beats of terror: hopping out of bed, exiting her safe bedroom, blasting her retinas with the shockingly bright bathroom light. And then, worst of all, she had to lift up the lid and unleash the horrifying creature surely waiting for her each time.

Later on, Forney made a comic about the commonly feared “demon in the toilet” and that comic found its way into the office of a pediatrician who uses it as a tool to help tell kids and parents that yes, children will probably be scared of things that...