Background: The aim of this cross-sectional study was to test the hypothesis that children diagnosed with nephroblastoma experience increased disturbance in renal filtration even after prompt treatment compared to patients treated for other neoplastic childhood diseases. Procedures: Our study included 127 children and young adults, who were successfully treated for nephroblastoma (n = 34), oncohaematological childhood diseases (n = 58), and other solid tumours (n = 35). In each patient, serum levels of cystatin C, microalbuminuria, and C-reactive protein, and serum and urine levels of creatinine were examined, along with a urine analysis. The estimated glomerular filtration rate (eGFR) was assessed using the Schwartz formula and Filler’s formula. Results: Our studies show that patients who were successfully treated for nephroblastoma and other solid tumours have lower GFR (GFRSch 118 ± 20, p = 0.00006, and 117 ± 22, p = 0.00003; GFRFiller 99 ± 17, p = 0.0001, and 104 ± 21 ml/min/1.73 m2, p = 0.0002) than children treated for oncohaematological diseases (GFRSch 137 ± 19, GFRFiller 121 ± 18 ml/min/1.73 m2). Conclusions: Patients diagnosed with nephroblastoma and other solid tumours have lower eGFR than children with oncohaematological childhood diseases and are at higher risk for developing CKD. This study demonstrates the necessity of nephrological monitoring.