Objectives: Indonesia has the second-highest number of tuberculosis (TB) cases globally, after India. The estimated TB incidence in Indonesia is 1 090 000 cases (387 cases per 100 000 population), with an associated mortality of 131 000 deaths (47 deaths per 100 000 population). The 2023–2024 TB Inventory Study in Indonesia aimed to quantify the extent of under-reporting by identifying TB cases that were diagnosed and/or treated but not reported to the National TB Surveillance System (Sistem Informasi Tuberkulosis [SITB]). This study specifically assessed the magnitude of under-reporting of TB cases among children younger than 15 years in Indonesia. Methods: This study used a cross-sectional design and analyzed data from the 2023–2024 TB Inventory Study (SIV-TB) in Indonesia. The sample included all TB cases in children younger than 15 years identified from eligible health facilities. Under-reported cases were defined as those that were diagnosed and/or treated but not reported to the SITB. Descriptive statistics were used to characterize under-reporting. A record-linkage process was performed to compare SIV-TB data with SITB data after deduplication. Results: A total of 12 293 TB cases in children younger than 15 years were identified across health facilities in 31 districts and cities. The overall under-reporting rate for childhood TB was 23.5%. The highest under-reporting rate was observed in Eastern Indonesia (28.0%). Conclusions Under-reporting of childhood TB was significantly associated with both age group and type of health facility