**Core Concept**
Graves' disease is an autoimmune disorder that leads to hyperthyroidism in children, caused by the production of autoantibodies against the thyroid-stimulating hormone (TSH) receptor, resulting in excessive thyroid hormone production.

**Why the Correct Answer is Right**
Graves' disease is the most common cause of thyrotoxicosis in childhood, accounting for approximately 90% of cases. This is due to the presence of autoantibodies that stimulate the TSH receptor, leading to increased thyroid hormone production. The increased levels of thyroid hormones can cause a variety of symptoms, including weight loss, palpitations, and exophthalmos. In children, Graves' disease can also lead to growth retardation and developmental delays if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxic nodular goitre is a rare cause of thyrotoxicosis in children, typically occurring in older adults. It is characterized by the presence of multiple nodules in the thyroid gland that produce excess thyroid hormones.

**Option B:** Toxic adenoma is a rare cause of thyrotoxicosis in children, typically occurring in adults. It is characterized by the presence of a solitary adenoma in the thyroid gland that produces excess thyroid hormones.

**Option D:** Thyrotoxicosis factitia is a rare cause of thyrotoxicosis in children, typically occurring in individuals who intentionally ingest excess thyroid hormone. This can be a form of Munchausen syndrome by proxy, where a caregiver intentionally administers thyroid hormone to a child.

**Clinical Pearl / High-Yield Fact**
Graves' disease is a classic example of an autoimmune disorder that can cause hyperthyroidism in children. It is essential to diagnose and treat Graves' disease promptly to prevent long-term complications, including growth retardation and developmental delays.

**✓ Correct Answer: C. Graves disease**