Final answer:
The symptom to be alert for in water intoxication is increased urine output.
Step-by-step explanation:
The symptom that the nurse would alert the parents of a child with diabetes insipidus to be on the lookout for in regards to water intoxication is increased urine output. Water intoxication occurs when there is excessive water retention in the body, leading to dilution of electrolytes. Increased urine output is a sign that the body may be retaining too much water and is unable to properly concentrate the urine, potentially causing an imbalance in electrolytes.