**Core Concept**
Bilateral proptosis in children is a clinical presentation that requires prompt identification of underlying causes. It involves the protrusion of the eyeballs, often due to increased orbital volume or pressure, which can be caused by various conditions such as thyroid ophthalmopathy, orbital tumors, or inflammatory diseases.

**Why the Correct Answer is Right**
The most common cause of bilateral proptosis in children is **Thyroid Ophthalmopathy** (also known as Graves' orbitopathy). This autoimmune disorder occurs in children with untreated hyperthyroidism, leading to inflammation of the orbital tissues and increased volume of the orbit. The resulting proptosis can cause discomfort, vision problems, and even corneal exposure. The pathophysiology involves the activation of immune cells and the release of cytokines and growth factors that stimulate the orbital tissues, leading to swelling and increased pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Orbital cellulitis is an infection of the orbital tissues, but it is less common in children and typically presents unilaterally.
**Option B:** Orbital tumors, such as rhabdomyosarcoma, can cause proptosis but are relatively rare in children and often involve a single orbit.
**Option C:** Cavernous sinus thrombosis can cause proptosis, but it is a more severe condition that typically presents with other symptoms such as cranial nerve palsies and is not as common as thyroid ophthalmopathy in children.

**Clinical Pearl / High-Yield Fact**
Thyroid ophthalmopathy is often associated with other autoimmune disorders, such as Hashimoto's thyroiditis, and can be a presenting feature of Graves' disease. It is essential to consider thyroid function tests and orbit imaging in children presenting with bilateral proptosis.

**Correct Answer:** C. Orbital tumors.