This study examines longitudinal changes in children's perceived usability based on two aspects. First, we developed a child-friendly usability questionnaire, which used cartoons to express the questionnaire response options. This approach provides an easy-to-understand five-point scale and a filling process using magnetic blocks, which together lead to highly reliable results. Additionally, we designed a longitudinal study to investigate the children's perceived usability according to two measurement methods (immediate and retrospective). The children's usability increased with longer durations of usage (i.e., increased repetitions of exercises). The short-term retrospective assessments depended on the most recent experience, whereas the long-term retrospective assessments were generally more positive. [ABSTRACT FROM AUTHOR]