Final answer:
The highest priority nursing action when a child is admitted with a low serum sodium level is instituting seizure precautions.
Step-by-step explanation:
The highest priority nursing action when a child is admitted to the pediatric unit with a serum sodium level of 118 mEq/L is instituting seizure precautions. Low serum sodium levels, also known as hyponatremia, can lead to neurological complications such as seizures. Therefore, it is crucial to ensure the child's safety by implementing seizure precautions, such as padding the bed and removing any potential hazards.