Pericardial effusion (PE) may require readmission after cardiac surgery and has been associated with postoperative morbidity and mortality. We sought to identify the prevalence and risk factors for postoperative PE requiring readmission in children. A retrospective analysis of the Pediatric Health Information System database was performed between January 1, 2003, and September 30, 2014. All patients ≤18 years old who underwent cardiac surgery were identified by ICD-9 codes. Those readmitted within 1 year with an ICD-9 code for PE were identified. Logistic regression analysis was performed to determine risk factors for PE readmissions. Of the 142,633 surgical admissions, 1535 (1.1%) were readmitted with PE. In multivariable analysis, older age at the initial surgical admission [odds ratio (OR) 1.17, p < 0.001], trisomy 21 (OR 1.24, p = 0.015), geographic region (OR 1.33–1.48, p ≤ 0.001), and specific surgical procedures [heart transplant (OR 1.82, p < 0.001), systemic-pulmonary artery shunt (OR 2.23, p < 0.001), and atrial septal defect surgical repair (OR 1.34, p < 0.001)] were independent risk factors for readmission with PE. Of readmitted patients, 44.2% underwent an interventional PE procedure. Factors associated with interventions included shorter length of stay (LOS) for the initial surgical admission (OR 0.85, p = 0.008), longer LOS for the readmission (OR 1.37, p < 0.001), and atrial septal defect surgery (OR 1.40, p = 0.005). In this administrative database of children undergoing cardiac surgery, readmissions for PE occurred after 1.1% of cardiac surgery admissions. The risk factors identified for readmissions and interventions may allow for improved risk stratification, family counseling, and earlier recognition of PE for children undergoing cardiac surgery.