Suellen M Walker1,2, Andrew Davidson3,4,5. 1. Pain Research, Developmental Neurosciences, UCL Institute of Child Health, London, UK. suellen.walker@ucl.ac.uk. 2. Department of Anaesthesia and Pain Medicine, Great Ormond Street Hospital NHS Foundation Trust, London, UK. suellen.walker@ucl.ac.uk. 3. Melbourne Children's Trials Centre, Murdoch Children's Research Institute, Parkville, Victoria, Australia. 4. Department of Anaesthesia, Royal Children's Hospital, Parkville, Victoria, Australia. 5. Department of Paediatrics, University of Melbourne, Parkville, Victoria, Australia.