Final answer:
The nurse should obtain the patient's weight as a priority assessment before administering the IV solution.
Step-by-step explanation:
The nurse should perform the priority assessment of obtaining the patient's weight before administering the IV solution. This is because the prescribed solution contains potassium chloride, which can be dangerous if given in excessive amounts. By obtaining the patient's weight, the nurse can calculate the appropriate dosage of the IV solution and ensure that it is safe for the child.