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 evaluation of the renal function is carried out according to the clinical trial protocols and to the center practices. To date, the standard methods (eg. creatinine clearance), as well as the available predictive formula (eg. Schwartz formula) are not well adapted to monitor children renal function. Indeed, the reliability of these methods depends on several parameters such as the diet and the muscle mass and could be very restrictive to perform on children. To circumvent these practical difficulties, the investigators intend to use the cystatin C as a biological marker of renal toxicity in children treated with cisplatin and / or ifosfamide. This cysteine protease has witnessed an upsurge of interest as an endogenous glomerular filtration rate marker and could be a good candidate to assess tubular toxicity when measured in urine.