Final answer:
The nurse should identify decreased serum sodium levels as an indication of SIADH. Other signs and symptoms include excessive thirst and weight gain.
Step-by-step explanation:
The nurse should identify decreased serum sodium levels as an indication that the child is developing Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH). SIADH is a condition where the body produces too much antidiuretic hormone (ADH), which causes the kidneys to reabsorb too much water, leading to dilutional hyponatremia (low serum sodium levels). Other signs and symptoms of SIADH include excessive thirst and weight gain due to fluid retention.