Children have proportionally more total body water (TBW) than adults: neonates ~75% TBW, infants ~65%, compared to ~60% in adults. A larger percentage exists in the extracellular compartment, making children more vulnerable to rapid fluid shifts. The immature kidneys have limited concentrating ability (maximum urine osmolality ~600 mOsm/kg in neonates vs. 1200 in adults), making them less able to compensate for fluid losses. Dehydration classification by tonicity is critical: isotonic (isonatremic, Na 130–150 mmol/L) is most common (~80%), hypotonic (hyponatremic, Na <130) causes cellular swelling and seizure risk, and hypertonic (hypernatremic, Na >150) requires slow correction to prevent cerebral edema. The nurse performs comprehensive fluid status assessment, calculates fluid deficits, manages IV rehydration protocols, monitors electrolytes, and recognizes signs of dehydration-related shock.