For the purpose of exploring the factors affecting injury severity of children and adolescents involved in traffic crashes in Greece, disaggregate crash data including 13,431 involving children and adolescents from all regions of Greece for the period 2006–2015 were utilized. In order to identify factors affecting injury severity and account for potential unobserved heterogeneity, a series of mixed logit models were utilized. To explore and address potential temporal instability of crash-related risk factors, the likelihood ratio test was applied. Results indicated that night crashes, crashes outside urban areas as well as crashes involving bicycles or powered-two-wheelers are associated with higher injury severity of children and adolescents. Interestingly, crashes involving pedestrians are associated with lower injury severity than head-on collisions and run-off-road collisions with fixed objects. Side and sideswipe crashes also result in lower injury severities. The likelihood ratio test indicated that crash-related factors are instable when comparing the models utilizing data before and after 2010 respectively. This study contributes to the current knowledge in the field, as to the best of our knowledge this is the first study that addresses unobserved heterogeneity when analyzing child and adolescent injury severity. Overall, the findings of this study provide useful insights and could assist in unveiling crash risk factors and prioritize programs and measures to promote road safety of children and adolescents.