Geographic disparities in access to secondary pediatric care remain a significant issue in countries with universal health coverage, including South Korea. Our study investigated the relationship between the geographic availability of secondary pediatric care services and mortality rates among children and adolescents in South Korea. We analyzed district-level data to assess the percentage of those aged 0-19 years residing outside of a 60-minute travel radius from the nearest secondary pediatric care provider (Accessibility Vulnerability Index, AVI). This analysis was conducted in conjunction with national mortality statistics for the same age group spanning the years 2017 to 2021. The concentration index was used to evaluate socioeconomic disparities in the AVI among districts. Using generalized estimating equation models, we calculated the relative risk (RR) of annual mortality per 10% increase in the AVI for secondary pediatric care. The AVI ranged from 0% to 100% across the districts for the study period. The CI was -0.30 (95% CI, -0.41 to -0.19) in 2017 and -0.41 (95% CI, -0.52 to -0.30) in 2021, indicating that the proportion of those who could not access care within 60 minutes was disproportionately higher in districts with lower socioeconomic status. Our findings revealed an 8% rise in mortality rates among individuals aged 0-19 years for every 10% increase in AVI (95% CI, 1.06-1.10). The association between AVI and mortality was more pronounced during the COVID-19 pandemic (RR, 1.11; 95% CI, 1.08-1.14) than in the pre-pandemic period (RR< 1.07; 95% CI, 1.05-1.09; p for heterogeneity = 0.016). Significant disparities were found in geographic access to pediatric care, which were relevant for child survival. Achieving true universal health coverage requires an emphasis on timely access to care for all pediatric populations.