The estimated continuous cardiac output (esCCO) system is a hemodynamic monitor that uses electrocardiograms and pulse oximeter waves to noninvasively estimate cardiac output. The coefficients for esCCO measurement have been established for adult patients, but the appropriate coefficients for pediatric patients are unclear. Therefore, this study determined esCCO coefficients for pediatric patients and validated the accuracy and tracking ability of a modified esCCO system. An initial study compared cardiac output measurements using transthoracic echocardiography and esCCO in 60 pediatric patients aged <15 years who underwent elective noncardiac surgery. Consequently, the coefficients for the esCCO measurements were redefined for pediatric patients. The main study compared cardiac output measurements between transthoracic echocardiography and modified esCCO in 80 pediatric patients. Measurements were performed pre- and postoperatively, and the accuracy and trending ability of the cardiac output measurements were evaluated using Bland-Altman analysis and a polar plot. The correlation coefficients between the modified esCCO and transthoracic echocardiography were 0.96 and 0.98 in the pre- and postoperative measurements, respectively. In Bland-Altman analysis, the bias (standard deviation [SD]), 95% limits of agreement, and percentage error were 0.03 (0.28), -0.53 to 0.60, and 18% in the preoperative measurement, and -0.04 (0.19), -0.42 to 0.35, and 15% in the postoperative measurement, respectively. The polar plot showed that the cardiac output changes were well tracked, with an angular bias (SD) of 2.9° (6.0°) and radial 95% limits of agreement ranging from -9.2° to 14.9°. Cardiac output measurement by esCCO with modified coefficients for pediatric patients showed high accuracy and tracking ability compared with cardiac output measurement by transthoracic echocardiography. This noninvasive cardiac output measurement could benefit perioperative hemodynamic monitoring in children.