The purpose of this study is to learn more about the feasibility of oral ketamine for the treatment of painful sickle-cell crises in children and adolescents as a supplement to intravenous (IV) opioids. There is a need for improved non-opioid analgesia for patients experiencing sickle-cell crises in the hospital and prehospital setting, as children and adolescents with sickle cell disease who experience sickle-cell crises often have severe pain that is not well controlled by high dose opioids, leading to poor pain management and opioid-related side effects. The study will begin when patients are admitted to the Emergency Department of Boston Children's Hospital for treatment of a sickle-cell crisis. Oral ketamine will be administered every 8 hours for the next 48 hours. Patients will have continuous cardiorespiratory monitoring for the duration of the study, as per routine care, as well as monitoring by the hospital's Acute Pain Service at least twice daily for pain management and side effects of pain treatment. At the end of the 48-hour study duration, patients will discuss with the Pain Service and Hematology Service whether to continue oral ketamine, change to intravenous ketamine, or discontinue ketamine based on clinical indications such as level of pain and sedation while on opioids.