Children who are undergoing surgery need intravenous fluids to prevent dehydration and maintain their electrolyte balance. The current standard of care in these children is to use a fluid which is low in sodium (hypotonic fluid). The safety of this practice has never been tested. There is ongoing concern from the medical community that this type of fluid increases the child's risk of developing low sodium levels, and hence may not be safe for all children. Low sodium can lead to significant complications such as seizures, coma and even death, risks of which are often underestimated and not anticipated by their caregivers. Experts in the field suggest that giving a solution with a similar sodium content to that of blood (isotonic fluid) reduces the risk of these problems in these children. This study will compare these two types of intravenous fluids (hypotonic versus isotonic), in a blinded fashion (i.e. neither the patient nor caregivers or investigators will be aware which type of fluid the patient is receiving), in children following surgery. The investigators goal is to see which type of fluid is safer, and leads to more stable sodium levels. This would in turn lead to a lower risk of complications as described above.