Purpose: Complete resection is the treatment of choice for most pediatric brain tumors, but early postoperative MRI for detection of residual tumor may be misleading due to magnetic resonance imaging (MRI) signal changes caused by the operation. PET (positron emission tomography) imaging with amino acid tracers in adults increase the diagnostic accuracy for brain tumors but the literature in pediatric neurooncology is limited. A hybrid PET/MRI system is highly beneficial in children reducing the number of scanning procedures and this is to our knowledge the first larger study using PET/MRI in pediatric neurooncology. We evaluated if additional postoperative 18F-fluoro-ethyl-tyrosine (18F-FET) PET in children and adolescents would 1) improve diagnostic accuracy for the detection of residual tumor as compared to MRI alone, and 2) assist clinical management. Methods: Twenty-two patients (7 males, mean age 9.5 years, range 0-19) were included prospectively and consecutively in the study and had twenty-seven early postoperative 18F-FET PET performed preferentially in a hybrid PET/MRI system (NCT03402425). Results: Using follow-up (93%) or re-operation (7%) as reference standard, PET combined with MRI discriminated tumor from treatment effects with a lesion based sensitivity/specificity/accuracy (95% confidence intervals) of 0.73(0.50-1.00)/1.00(0.74-1.00)/0.87(0.73-1.00) compared to MRI alone: 0.80(0.57-1.00)/0.75(0.53-0.94)/0.77(0.65-0.90), i.e. the specificity for PET/MRI was 1.00 as compared to 0.75 for MRI alone (P = 0.13). In 11 of 27 cases (41%), results from the 18F-FET PET scans added relevant clinical information including one scan that directly influenced clinical management as an additional residual tumor site was identified. 18F-FET uptake in reactive changes was frequent (52%) but correct interpretation was possible in all cases. Conclusion: The high specificity for detecting residual tumor suggests that supplementary 18F-FET PET is relevant in cases where re-operation for residual tumor is considered.