This trial has been designed by clinicians, working in Starship Children’s Hospital, in response to recent data suggesting hospital-wide protocols for intravenous (IV) fluid therapy given to children must shift from our current standard of hypotonic (low sodium) to isotonic (physiological sodium) fluids to avoid hyponatraemia. When altering protocols to give isotonic fluids to children as standard, the options are to administer supraphysiological amounts of chloride (0.9% saline), or a more physiological, but more expensive, balanced solution (Plasmalyte ® 148). 0.9% saline has been used for many years in children as the standard resuscitation fluid and for maintenance fluids when isotonic fluids are indicated. However, a growing body of evidence in adults suggests that the extra chloride might contribute to acute kidney injury. Retrospective evidence suggests that 0.9% saline might also increase length of stay in patients (including children) admitted with specific conditions.