Error-monitoring is a crucial cognitive process that enables us to adapt to the constantly changing environment. The anterior cingulate cortex (ACC) plays a vital role in error-monitoring, and its prolonged maturation suggests that it can be influenced by experience-dependent plasticity. To explore this possibility, we collected morphometric magnetic resonance imaging (MRI) measures of the ACC and error-related response-locked event-related potentials (ERPs) in 26 schoolchildren, aged 6–12 years, enrolled in either a Montessori or a traditional curriculum in Switzerland. We show that the caudal ACC undergoes significant morphometric changes during this developmental age range that seem related to error detection ERP activity. Furthermore, we observed differences in source localization activity related to error detection within the caudal ACC between Montessori and traditionally-schooled children, indicating a potential difference in the development of error-monitoring in these groups. Our study provides preliminary evidence for a potential window of opportunity to influence error-monitoring during development and calls for more work in that direction.