Transient Synovitis: This is the most common cause of hip pain in children, often presenting with sudden onset of pain and limited mobility, usually resolving on its own within a few days. It's more common in children under the age of 10.

Slipped Capital Femoral Epiphysis (SCFE): A condition where the ball at the upper end of the femur (thigh bone) slips out of place, more common in obese adolescents. It presents with hip, thigh, or knee pain and is usually associated with a limp.
Legg-Calve-Perthes Disease: A condition characterized by the temporary loss of blood supply to the hip, leading to the softening and breakdown of the bone. It typically affects children between the ages of 4 and 8 and presents with hip pain and a limp.
Septic Arthritis: A bacterial infection within a joint, which is a medical emergency. Symptoms include severe pain, swelling, redness, and warmth around the joint, along with fever and chills.

Malignancy (e.g., Osteosarcoma, Ewing's Sarcoma): Although rare, bone cancers can present with hip pain and must be considered to ensure timely diagnosis and treatment.
Osteonecrosis: The death of bone tissue due to a lack of blood supply, which can be caused by various factors including trauma, steroids, and sickle cell disease.
Juvenile Idiopathic Arthritis (JIA): An autoimmune disease that can cause joint pain and swelling, including in the hips, and if left untreated, can lead to significant morbidity.

Gaucher's Disease: A genetic disorder that can lead to bone pain and other systemic symptoms due to the accumulation of harmful quantities of a certain fatty substance (lipid) throughout the body.
Multiple Epiphyseal Dysplasia: A rare genetic disorder affecting the ends of bones, leading to skeletal malformations and potential hip pain.
Sickle Cell Disease with Avascular Necrosis: While not rare in populations with a high prevalence of sickle cell disease, it's a condition where the abnormal sickle-shaped red blood cells can lead to avascular necrosis of the bone, causing hip pain.