Cisplatin and ifosfamide are commonly used drugs in chemotherapy. They are known to involve renal toxic threats in children given their immature kidney. This toxicity is increased especially after nephrectomy and/or concomitant radiotherapy. In pediatric oncology, the available evaluation methods of the renal function could be very restrictive to perform on children. In this study, the investigators intend to test the use of the cystatin C as an effective and reliable biological marker of renal toxicity in children treated with cisplatin and / or ifosfamide.