The purpose of this study was to examine the relationship between the Child Occupational Self-Assessment (COSA) and quality of life (QoL) in elementary school children. A total of 77 children from a public elementary school were included in the study. A survey was conducted using the COSA and the Kid-KINDLR. The COSA consists of 25 items on occupational competence and value. The items are classified into four factor structures: challenging occupation, motivated occupation, daily life tasks, and expected tasks. The Kid-KINDLR is a scale consisting of 24 items in question form by which the QOL of children can be grasped in multiple dimensions. Groups that obtained high and low QoL levels showed a significant difference in COSA regarding the value score obtained in the challenging occupation factor. Tackling, trying to solve, or struggling with difficult tasks may improve QoL. Children actively working on mandatory activities, rather than doing given assignments, appear to have higher QoL.