Final answer:
Hyperpigmentation and low blood pressure are the assessment findings most indicative of Addison's disease in this scenario.
Step-by-step explanation:
The assessment finding most indicative of Addison's disease in the 9-year-old girl would be hyperpigmentation and low blood pressure. Addison's disease is characterized by insufficient cortisol production from the adrenal gland. This can result in symptoms such as low blood pressure, hyperpigmentation of the skin, extreme weakness, fatigue, and loss of appetite. Therefore, the presence of hyperpigmentation and low blood pressure would be most suggestive of Addison's disease in this case.