Epididymitis: This is the most common cause of a swollen testicle in children, especially in prepubertal boys. It is an inflammation of the epididymis, usually caused by a bacterial infection.

Testicular torsion: Although less common than epididymitis, testicular torsion is a significant concern due to the potential for testicular loss if not promptly treated. It occurs when the spermatic cord becomes twisted, cutting off the blood supply to the testicle.
Hydrocele: A hydrocele is an accumulation of fluid in the tunica vaginalis, a potential space around the testicle. It can cause the testicle to appear swollen.
Varicocele: A varicocele is an enlargement of the veins within the scrotum, similar to varicose veins. It can cause swelling and discomfort.

Testicular cancer: Although rare in children, testicular cancer is a critical diagnosis not to miss. It can present as a painless swelling or mass in the testicle.
Trauma: Traumatic injury to the testicle can cause swelling and pain. It's essential to consider the history of trauma when evaluating a swollen testicle.

Henoch-Schönlein purpura (HSP): HSP is a form of vasculitis that can cause testicular swelling, among other symptoms like purpura, joint pain, and abdominal pain.
Sarcoidosis: Sarcoidosis is a rare condition that can cause granulomatous inflammation in various parts of the body, including the testicles, leading to swelling.
Tuberculosis: Tuberculosis (TB) can affect the testicles, causing swelling and other symptoms like pain and discharge. It's more common in areas where TB is prevalent.