Nausea and Vomiting in Postoperative Paediatric Patients With Patient-Controlled Analgesia (PCA): Morphine vs Oxycodone

POPCORN trial will compare the side effects and effectiveness of Morphine versus Oxycodone medication when prescribed for use as patient controlled analgesia (PCA) for pain relief for paediatric patients after-surgery. This trial is embedded into routine patient care using the hospital electronic medical record (EMR). Participants will be randomly assigned to either medication after they enrol in the study. The main questions the POPCORN trial aims to answer are: * 1\. Is there a difference in the usage of medication to treat nausea and vomiting for those who received oxycodone PCA versus morphine PCA for post-surgery pain relief? * 2\. Is there a difference in side effects or pain relief needed between the two groups? Study activities are as follows: * Participants enrolled to study during their pre-operative consultation * Participants are randomly assigned to morphine or oxycodone * No further study-specific activities expected from participant after enrolment and randomisation * Participant receives routine medical care as planned * Clinicians record assessments as per routine care in electronic medical record (EMR) * EMR data are extracted as trial data