<qa>  What is paracetamol?The benefit of small doses of ketamine has been demonstrated for postoperative analgesia in adults but remains unproved in children. The investigators purpose is to evaluate the effects of continuous intravenous small doses of ketamine versus placebo to improve the quality of postoperative analgesia in children 6 months to 6 years of age. Caudal anesthesia is performed for intraoperative analgesia and all children receive paracetamol, a non-steroidal anti-inflammatory and continuous intravenous nalbuphine. 