**Core Concept**
Orbital metastasis in children is a rare but serious condition, often resulting from the spread of cancer from another primary site. The most common primary cancers that metastasize to the orbit in children are neuroblastoma and retinoblastoma.

**Why the Correct Answer is Right**
The correct answer is neuroblastoma, which is a type of cancer that arises from the neural crest cells of the sympathetic nervous system. Neuroblastoma is known to metastasize to various sites, including the orbit, due to its tendency to spread through the bloodstream and lymphatic system. The high vascularity of the orbit and the presence of neural crest cells make it a common site for neuroblastoma metastasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoblastoma is a primary intraocular malignancy that rarely metastasizes to the orbit. While it is a common intraocular tumor in children, it is not the most common cause of orbital metastasis.
**Option B:** Leukemia can cause orbital metastasis, but it is less common than neuroblastoma. Leukemic infiltration of the orbit is often seen in the context of acute lymphoblastic leukemia (ALL).
**Option C:** Lymphoma is a rare cause of orbital metastasis in children, and when it occurs, it is often associated with systemic lymphoma.

**Clinical Pearl / High-Yield Fact**
Neuroblastoma is the most common extracranial solid tumor in children, and it often presents with metastasis to the orbit, bone, or bone marrow.

**Correct Answer:** C.