A 3-year-old child presents to the emergency department with signs of septic shock: fever (39.5°C), tachycardia (HR 160/min), hypotension (BP 70/40 mmHg), delayed capillary refill (>2 seconds), and altered mental status. After rapid fluid resuscitation with 20 mL/kg normal saline bolus over 15 minutes, the child remains hypotensive and shows signs of poor perfusion. What is the first-line vasopressor agent of choice in pediatric septic shock?