**Core Concept**
Orbital tumors in children can be benign or malignant, arising from various tissues including bone, soft tissue, and vascular elements. The most common orbital tumors in children often have distinct histopathological features and clinical presentations.

**Why the Correct Answer is Right**
The most common orbital tumor in children is the rhabdomyosarcoma. This malignant tumor arises from the embryonal muscle cells and is characterized by its rapid growth and potential for metastasis. Rhabdomyosarcoma typically presents with proptosis, swelling of the eyelids, and diplopia due to mechanical compression of the surrounding orbital structures. The tumor often expresses the muscle-specific actin and myogenin proteins, which are useful markers for its diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lymphoma is a type of orbital tumor, but it is relatively rare in children and usually presents with systemic symptoms and lymphadenopathy.
* **Option B:** Hemangioma is a common benign vascular tumor, but it is typically seen in adults and is characterized by its slow growth and lack of systemic symptoms.
* **Option C:** Dermoid cyst is a congenital anomaly that can present as an orbital tumor, but it is usually benign and presents with a well-defined mass and skin appendages.

**Clinical Pearl / High-Yield Fact**
Rhabdomyosarcoma is the most common primary orbital malignancy in children, and its diagnosis requires a combination of clinical evaluation, imaging studies, and histopathological examination. The tumor's rapid growth and potential for metastasis necessitate prompt treatment with chemotherapy, radiation, and surgery.

**Correct Answer: A. Rhabdomyosarcoma**