This cross-sectional study examined associations between physical and psychological health indicators in primary school children, focusing on constructs that have received less attention in previous models. In addition to muscular fitness, physical self-concept, and mental health—represented by self-compassion and psychological wellbeing—posture was included as an additional health variable to explore its potential relevance within this framework. A total of 174 children aged 6–11 years participated in the observational study. Standardized measures were used to assess muscular fitness, posture, psychological wellbeing, physical self-concept, and self-compassion. Associations between variables were examined using structural equation modeling with 12 manifest variables. The hypotheses targeted positive correlations within and between the physical and psychological domains. The results confirmed a strong association between muscular fitness and posture. However, no significant links were found between posture or muscular fitness and psychological wellbeing, self-compassion, or physical self-concept. In contrast, positive associations emerged between psychological wellbeing, positive self-compassion, and physical self-concept. These findings suggest that psychophysical relationships may not yet be fully established or measurable at this developmental stage. Future research should investigate the developmental trajectories of these relationships in more diverse populations. However, the present findings must be interpreted with caution due to the high parental educational level in the sample.