Hard lumps in the breasts of a six-year-old girl can be caused by precocious puberty, trauma and other reasons. 1. Precocious puberty: If a six-year-old girl has hard lumps in her breasts, accompanied by breast enlargement and pain, most of the time it is due to precocious puberty. 2. Other: when the breast is traumatized, local staphylococcus aureus infection and other factors cause inflammation, which can lead to the symptoms of six-year-old girls with breast lumps. When the symptoms appear, parents are advised to take their children to the hospital in time, in order to early formal treatment, to avoid affecting the healthy growth and development of the child, do not use their own medicine or hard to rub.