**Core Concept**
Sinusitis in children often affects the ethmoid sinus due to its unique anatomical structure and development. The ethmoid sinus is a complex, air-filled cavity located between the nasal cavity and the eyes, consisting of multiple small cells that are prone to infection.

**Why the Correct Answer is Right**
The ethmoid sinus is more susceptible to sinusitis in children due to its extensive pneumatization, which increases the surface area for bacterial colonization. The ethmoid sinus also has a rich blood supply and is surrounded by thin bone, making it more vulnerable to infection. Additionally, the ethmoid sinus is closely associated with the nasal cavity, allowing for easy spread of pathogens from the upper respiratory tract.

**Why Each Wrong Option is Incorrect**
**Option A:** The frontal sinus is less common in children and typically develops later in life, making it a less likely site for sinusitis in this age group.

**Option B:** While the maxillary sinus can be affected by sinusitis, it is less common in children compared to the ethmoid sinus. The maxillary sinus is also more prone to complications like orbital cellulitis due to its proximity to the orbit.

**Option D:** The sphenoid sinus is the least likely to be affected by sinusitis in children, as it is a smaller, more posterior sinus that develops later in life.

**Clinical Pearl / High-Yield Fact**
The "triangle of pain" in sinusitis refers to the area bounded by the medial canthus, the lateral canthus, and the root of the nose. This area is often tender to palpation in cases of ethmoid sinusitis.

**✓ Correct Answer: C. Ethmoid**