 Children with cerebral palsy commonly undergo multilevel surgery, meaning several lower limb combined procedures performed during the same surgical intervention. The aim of this type of surgery is to correct all deformities together in order to restore near to normal anatomy and muscular function. It is very important to be able to obtain good range of motion after surgery, in order to consolidate surgical results. During the first days after the operation, children are sore and it may be difficult to realize adequate physical therapy. In order to palliate this situation, MEOPA gaz is used during REHAB sessions. Good results have been obtained so far but no study is available to demonstrate these results. The goal of our research is to prove that there is a benefit in using MEOPA postoperatively in these patients. 