Efficiency of IV Dexamethasone, Administered After a Lower Limb Blockade, on the Post Operative Pain in Children

Intravenous dexamethasone (IV) can be used alone in adults as an adjunct to peri-nerves blocks, for diffusion blocks. In pediatrics, dexamethasone is used daily according to the assessment of the anesthetist, by extension of the recommendations of the adult, as adjuvant in perinervous blocks although no study has been published yet concerning his interest. However, the physiology of the child is not superimposable to that of the adult (renal function, volume of distribution, plasma protein binding ...). We thus wish to study the effectiveness of the administration of IV dexamethasone at the time of anesthetic induction in the prolongation of the duration of the block of the lower limbs in the child and its repercussion on the postoperative consumption of morphine. Main Objective : Evaluate the efficiency, compared to placebo, of IV dexamethasone at the dose of 0.2 mg / kg administered as a bolus at the time of anesthetic induction, on the early postoperative pain (first 24 hours) in the child of 6 to 15 years undergoing surgery that requires the achievement of a peri-neural block of the lower limb after general anesthesia Secondary objectives : * Evaluate the efficiency of 0,2mg/kg IV dexamathasone administrated at anesthesia induction compared to placebo, on the increase of the delay of first post operative morphinic consumption in children from 6 to 15 who are qualified for a lower limb surgery with a loco-regional anesthesia. * Evaluate that 0,2mg/kg IV dexamathasone administrated at anesthesia induction compared to placebo, doesn't increase the lower limb motor blokade duration in children from 6 to 15 who are qualified for a lower limb surgery with a loco-regional anesthesia * Evaluate the efficiency of 0,2mg/kg IV dexamathasone administrated at anesthesia induction compared to placebo, on the decrease of post operative nausea and vomiting in children from 6 to 15 who are qualified for a lower limb surgery with a loco-regional anesthesia Principal endpoin