Lipoma: A common benign tumor of fat cells, which can occur anywhere on the body, including the lumbar area. It's often soft, movable, and painless, making it a likely diagnosis for a soft tissue mass in a child.

Hemangioma: A benign vascular tumor that can present as a soft tissue mass. It's common in children and can appear anywhere on the body.
Neurofibroma: A benign nerve sheath tumor associated with neurofibromatosis type 1 (NF1). It can present as a soft, palpable mass and is more likely in children with a family history of NF1.
Pilomatricoma: A benign skin tumor that originates from the hair follicle matrix. It can feel like a hard or soft mass, depending on its composition.

Rhabdomyosarcoma: A malignant tumor of muscle origin, which is the most common soft tissue sarcoma in children. It requires prompt diagnosis and treatment due to its aggressive nature.
Ewing's Sarcoma: A highly malignant bone tumor that can also present as a soft tissue mass. It's rare but critical to diagnose early due to its aggressive behavior and potential for metastasis.
Wilms Tumor: A kidney cancer that can present as a palpable abdominal mass, which might be felt in the lumbar area. Although it primarily involves the kidney, it's crucial to consider due to its severity.

Desmoid Tumor: A rare, benign but locally aggressive tumor that can occur in the abdominal wall or retroperitoneum, potentially presenting as a lumbar mass.
Lymphangioma: A rare, benign tumor of the lymphatic vessels, which can present as a soft tissue mass. It's more common in the neck and axilla but can occur in other areas.
Teratoma: A rare tumor that can contain several different types of tissues, including hair, muscle, and bone. It can present as a mass in various locations, including the lumbar area, though it's more commonly found in the sacrococcygeal region or gonads.