Sacroccoccygeal Teratoma (SCT): This is the most common diagnosis for a pre-sacral mass in a child, especially given the age and location. SCTs are congenital tumors that can grow to a significant size and may be detected prenatally or shortly after birth.

Neuroblastoma: Although more commonly associated with the adrenal glands, neuroblastoma can also arise from sympathetic nervous system tissues in the pelvis, presenting as a pre-sacral mass.
Rhabdomyosarcoma: A type of soft tissue sarcoma that can occur in various parts of the body, including the pelvis, and present as a mass.
Pelvic Neurofibroma: Part of neurofibromatosis type 1 (NF1), these benign nerve sheath tumors can occur anywhere along the nervous system, including the pre-sacral area.

Malignant Germ Cell Tumor: Although less common, these tumors can be aggressive and require prompt treatment. Their presence in the pre-sacral area, though rare, necessitates consideration due to their potential for malignancy.
Ewing's Sarcoma: A highly malignant bone tumor that can occasionally present in the pelvis and could be mistaken for a soft tissue mass.

Chordoma: A rare, slow-growing malignant tumor that originates from the notochord, often found in the sacrum or coccyx, which could present as a pre-sacral mass.
Meningeal Cyst: A rare congenital anomaly that can present as a pre-sacral mass due to herniation of the meninges through a defect in the sacrum or coccyx.
Anterior Sacral Meningocele: A rare condition where there is a herniation of the meninges through a defect in the anterior aspect of the sacrum, which can present as a pre-sacral mass.