**Core Concept**
Chronic periostitis in children refers to a long-standing inflammation of the periosteum, a layer of connective tissue surrounding bones. This condition often results from chronic infection, trauma, or other inflammatory processes. The periosteum plays a crucial role in bone healing and remodeling.

**Why the Correct Answer is Right**
Chronic periostitis in children is known as Ewing's sarcoma. Ewing's sarcoma is a rare type of bone cancer that typically affects children and young adults. It arises from the periosteum and can cause significant bone destruction and periosteal reaction. The exact cause of Ewing's sarcoma is unknown, but it is believed to result from a combination of genetic and environmental factors.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteochondritis dissecans is a condition characterized by the loosening of a piece of cartilage and bone from the surrounding bone, but it is not a form of chronic periostitis.
**Option B:** Osteomyelitis is a bacterial infection of the bone, but it is not a specific term for chronic periostitis in children.
**Option C:** Fibrous dysplasia is a non-cancerous condition that affects the bone and causes it to become soft and prone to fractures, but it is not a form of chronic periostitis.

**Clinical Pearl / High-Yield Fact**
Ewing's sarcoma often presents with a characteristic "onion skin" appearance on radiographs due to the periosteal reaction, which is a thickening of the periosteum in response to the tumor.

**Correct Answer: D. Ewing's sarcoma.**