In ball games, knowledge of tactics is required both during a match and while planning operations before a match. Therefore, for children to learn to play ball games effectively, knowledge on tactics is important. This study aimed to clarify how children’s tactical knowledge changed following a unit focusing on the offensive guard in flag football. To this end, a tactical knowledge test was conducted before and after the unit, and the results were analyzed. Findings showed that many children had acquired tactical knowledge related to offensive guards. Children gained two specific types of knowledge. First, they gained declarative knowledge about offensive guards, such as the role of the guard in the football game. Second, the gained procedural knowledge about offensive guards, such as how guards must remain conscious of the movements of opponents and teammates. Additionally, many children acquired procedural knowledge regarding the guarding tactics necessary to make plays successful.