Background: On October 1, 2017, a new coinsurance reduction policy for children under 15 was introduced to minimize the lack of inpatient medical services for economic reasons and secure children’s access to medical care. Methods: This study analyzes the effect of this coinsurance reduction policy on healthcare utilization using data from the National Health Insurance Service-National Sample Cohort between 2015 and 2019. Groups were classified by 3 case groups and a control group according to age. The dependent variables were inpatient cost, admission, length of hospitalization, outpatient cost and visit, and total cost. The difference-in-differences method was used to examine changes in healthcare utilization among the case and control groups after policy implementation. Results: Children of the age group 1–5 exhibited an increase in inpatient services and a decrease in outpatient services. There was a 16.17% increase in inpatient cost, 8.55% increase in inpatient admission, 10.67% increase in inpatient length of hospitalization, −9.14% decline in outpatient cost, and −6.79% decline in outpatient visits. Regarding children in the age groups of 6–10 and 11–15, the effect of the policy was inconclusive. Conclusion The reduction in coinsurance rate policy in hospitalization among children has increased inpatient services and reduced outpatient services for 1–5-year-olds—a substitute effect was observed in this group. There is need for further research to examine the longterm effects of the coinsurance reduction policy.