A Delphi study is conducted to develop items for a tool that measures the development of core competency in preschool children. Through a literature review and analysis the components and preliminary items were derived. A Delphi panel was formed with experts from academia and practice in fields such as early childhood education, psychology, and medicine, with this step followed by two rounds of Delphi surveys. The first Delphi survey confirmed the validity of the tool’s purpose and its components, leading to a process of deleting, retaining, or modifying items based on expert feedback. The second Delphi survey was then conducted using the restructured items. To assess item appropriateness, the average scores for each item and the content validity ratio (CVR) were analyzed. As a result, a total of 178 items were identified across four areas: 50 items for communication, 32 items for collaboration, 48 items for creativity, and 48 items for critical thinking. Ultimately, this study provides foundational data for establishing a valid assessment tool for core competency for preschool children through a Delphi analysis while also suggesting that future standardization research is necessary.