A 4-year-old boy is admitted to the pediatric ward 6 hours after immersion in a pot of boiling water. He has scalding injuries over his bilateral lower limbs and buttocks with a total body surface area (TBSA) involvement of 18%. The affected skin shows blistering with a deep red base, minimal blanching, and markedly diminished pain sensation. The child is restless and tachycardic (HR 130/min). What is the most appropriate immediate management priority?