Children with a hard bag on the head may be caused by folliculitis, trauma, sebaceous cysts and other factors. 1. Folliculitis: the disease is usually caused by inflammation of the hair follicles due to infection by bacteria, fungi and other pathogens, patients may have symptoms of swelling around the hair follicles, which may also be accompanied by localized reddening of the skin and elevated temperature. 2. Trauma: When the head tissue is traumatized by collision, blows and other traumatic injuries, it can lead to tissue oozing or subcutaneous bleeding, resulting in a hard bag growing on the head of the child. 3. Sebaceous cysts: the disease is usually caused by the blockage of sebaceous gland excretory ducts, sebum stagnation in the sebaceous glands, the glands gradually expand to form lumps. It is recommended that the patient go to the hospital in time to find out the cause of the disease and then carry out the relevant treatment.