Constipation: This is a common issue in children that can cause pelvic pain due to the accumulation of stool in the rectum, which is located in the pelvis. The pain can be localized to one side if the constipation is more pronounced on that side.

Urinary Tract Infection (UTI): UTIs can cause pain in the pelvic area, especially if the infection involves the lower urinary tract. The pain can be unilateral if the infection is more severe on one side.
Musculoskeletal Injury: Children are prone to injuries from falls or sports activities, and a strain or sprain in the pelvic region could cause unilateral pain.
Appendicitis: Although appendicitis typically presents with right lower quadrant pain, it can occasionally present atypically, especially in children, with pain on the left side if the appendix is retrocecal and extends to the left.

Testicular Torsion: This is a surgical emergency that can cause severe, unilateral pain. Although it typically presents with testicular pain, it can radiate to the pelvic area.
Intussusception: A condition where a part of the intestine slides into another, which can cause severe abdominal and pelvic pain. It's more common in younger children but should be considered in any child with severe, unexplained abdominal pain.
Osteomyelitis or Septic Arthritis of the Hip: Infections of the bone or joint can cause severe pain and are medical emergencies. The pain can be referred to the pelvic area.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic pelvic pain, although they are less common in young children.
Neuroblastoma: A type of cancer that can arise in the adrenal glands or along the sympathetic nervous system and cause pelvic pain if it metastasizes to the pelvis.
Ewing's Sarcoma: A rare type of bone cancer that can cause pain in the affected bone, which could be in the pelvis.