Primary Polydipsia: This condition is characterized by excessive fluid intake leading to the production of large volumes of dilute urine. The history of being forced to drink excessive water and the low urine osmolality of 76 mosm/kg support this diagnosis. The child's weight and age are also consistent with this condition.

Diabetes Insipidus (Central or Nephrogenic): Although less likely given the context of forced water intake, diabetes insipidus could present with similar symptoms, including nocturnal enuresis and dilute urine. Central diabetes insipidus is due to a deficiency of antidiuretic hormone (ADH), while nephrogenic diabetes insipidus is due to renal insensitivity to ADH.
Psychogenic Polydipsia: This condition involves excessive water drinking due to psychological factors, which could be a consideration if the forced water intake is not the sole reason for the polydipsia.

Diabetes Mellitus: Although the primary symptom here is nocturnal enuresis with dilute urine, diabetes mellitus can present with polyuria and polydipsia. It's crucial to rule out diabetes mellitus due to its significant implications for health and management.
Renal Tubular Acidosis: Certain types of renal tubular acidosis can lead to an inability to concentrate urine, resulting in polyuria and nocturnal enuresis.

Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD): A rare condition where the kidneys inappropriately concentrate urine in response to ADH, but this would typically present with more concentrated urine, making it less likely in this scenario.
Bartter Syndrome or Gitelman Syndrome: These are rare genetic disorders affecting the kidneys' ability to reabsorb potassium, leading to an inability to concentrate urine. However, they are characterized by specific electrolyte imbalances and other symptoms not mentioned in the scenario.

Urine Specific Gravity: To further assess the kidney's ability to concentrate urine.
Blood Glucose: To rule out diabetes mellitus.
Electrolyte Panel: Including sodium, potassium, and chloride to assess for any imbalances suggestive of specific renal tubular disorders.
ADH Level: If central diabetes insipidus is suspected.
Water Deprivation Test: To assess the body's ability to concentrate urine when dehydrated, helpful in diagnosing diabetes insipidus.

Address the Forced Water Intake: The first step is to stop forcing the child to drink excessive water, as this is likely the primary cause of the nocturnal enuresis and dilute urine.
Fluid Management Education: Educate the child and caregivers on appropriate fluid intake.
Behavioral Therapy for Nocturnal Enuresis: Techniques such as bladder training, alarms, and motivational therapy can be helpful.
Treatment of Underlying Conditions: If other diagnoses are confirmed (e.g., diabetes insipidus, diabetes mellitus), specific treatments for those conditions should be initiated under medical supervision.