Hard knots under the skin of a child’s arm after a bump can be treated with hot and cold compresses and, if necessary, surgery. Hard knots under the skin after a child’s arm injury are mostly due to hematoma formed by local bleeding. Cold compresses can be applied with ice to induce capillary contraction and reduce bleeding after a bump. After three days, you can choose hot water bag hot compress, in order to promote blood circulation, promote the absorption of hematoma. If the hematoma is not absorbed for a long time, you can also choose surgical incision and drainage treatment, and after the surgery, oral amoxicillin and other antibiotics to prevent infection. If your child’s arm is bruised and hard knots appear under the skin, it is recommended to go to the hospital to improve the examination and receive regular treatment.