Urinary Tract Infection (UTI): This is a common cause of flank pain in children, especially in girls. The pain is usually accompanied by other symptoms such as dysuria, frequency, and fever.

Nephrolithiasis (Kidney Stones): Although less common in children than in adults, kidney stones can cause severe flank pain, often radiating to the groin, and can be accompanied by nausea, vomiting, and hematuria.
Pyelonephritis: An infection of the kidney, which can cause flank pain, fever, and other systemic symptoms. It's more common in younger children and those with underlying urinary tract abnormalities.
Musculoskeletal Pain: Strains or overuse injuries can cause flank pain in children, especially in those involved in sports or physical activities.

Appendicitis: Although the pain typically starts around the navel and then moves to the lower right abdomen, in some cases, especially if the appendix is retrocecal, the pain can be referred to the flank. Missing appendicitis can lead to serious complications.
Testicular Torsion: A surgical emergency that can sometimes present with flank pain if the testicle is undescended or if there's referred pain. It's crucial to examine the testes in any boy presenting with acute scrotal or flank pain.
Pneumonia: Lower lobe pneumonia can cause referred pain to the flank due to the proximity of the diaphragm to the lower lobes of the lungs. This is particularly important in younger children who may not be able to localize their pain accurately.

Wilms Tumor: A type of kidney cancer that can cause a palpable mass and flank pain in children, typically presenting in children under the age of 5.
Polycystic Kidney Disease (PKD): A genetic disorder that can cause cysts to form on the kidneys, potentially leading to flank pain, although it's more commonly diagnosed in adults.
Sickle Cell Disease: Can cause flank pain due to sickling crises affecting the kidneys or due to other complications like pyelonephritis or nephrolithiasis.