**Core Concept**
Acute sinusitis in children often involves the ethmoid sinuses, which are a group of small air-filled cavities located between the nose and the eyes. The ethmoid sinuses are a common site of infection in children due to their small size and rich blood supply.

**Why the Correct Answer is Right**
The ethmoid sinuses are more prone to infection in children due to their anatomy and physiology. The ethmoid sinuses are separated from the nasal cavity by a thin mucous membrane, making them more susceptible to bacterial and viral infections. Additionally, the ethmoid sinuses are richly supplied with blood vessels, which can lead to increased inflammation and congestion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Maxillary sinuses are more commonly involved in adults with acute sinusitis, not children.
* **Option B:** Frontal sinuses are less commonly involved in acute sinusitis in children due to their larger size and more complex anatomy.
* **Option D:** Sphenoid sinuses are the least commonly involved sinuses in acute sinusitis, especially in children.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presentation of acute sinusitis in children can be nonspecific, with symptoms such as nasal congestion, cough, and fever. A high index of suspicion is required to diagnose sinusitis in children, and imaging studies such as CT scans may be necessary to confirm the diagnosis.

**Correct Answer: C. Ethmoid sinuses.**