A 2-year-old child is brought to the Emergency Department with a 1-day history of fever (39.5°C), irritability, and refusing to eat or drink. On examination, the child is lethargic, has a heart rate of 160 bpm, respiratory rate of 40/min, and a capillary refill time of 3 seconds. A non-blanching rash is noted on their trunk and limbs. What is the single most important immediate action?