In a child less than 2 years old, which type of mass is described as soft, non-tender, and not translucent?

In children under 2 years old, the features of a mass that is soft, non-tender, and not translucent point towards a branchial cyst. Branchial cysts are typically found along the neck's lateral aspects and are the result of the remnants of developmental structures from embryonic development. They are usually soft to the touch due to their cystic nature and lack of surrounding inflammation, which accounts for the non-tender quality.

These cysts are not translucent, which differentiates them from other types of cystic lesions such as lymphangiomas that may exhibit a more fluid-filled, translucent appearance. Teratomas typically present as solid masses containing different types of tissues and can be either tender or non-tender depending on their location and associated complications. Thyroglossal duct cysts, while also soft and non-tender, may present midline and often exhibit some level of translucency, particularly if they are filled with fluid. Given these considerations, the characteristics of the mass in the presented scenario align closely with those of a branchial cyst.