This article presents how a timeline can be used to improve the quality of qualitative and interdisciplinary child studies research with a focus on documents and archives. A particular case, Swedish school sex education from the 1880s until 2022, is used to present and describe the advantages of applying what we refer to as timeline-thinking practices. We, the authors, from two interdisciplinary disciplines, reflect upon our own research process during a four-year project, in which we have interpreted many document sources and actors with the help of timelines. We describe timeline-thinking practices focused on empirical sources and interpretation as a joint, collaborative, time-consuming, creative, and engaging activity. Periodization was used as a starting point, and we explore how year-triggering periodization and concept-triggering periodization actualized varying versions of school sex education. Despite the chronological structure of timelines, they forced us to move back and forth in time, which improved the quality of the joint interpretations and led to truly collaborative work. Hence, the timeline-thinking practices we employed helped us to both identify more connections and handle messiness in a systematic way; they encouraged deep thinking and reflection, and improved the reliability of the interdisciplinary research.