A child’s legs can be bruised and purple by trauma, thrombocytopenia, allergic purpura and other causes. 1. Trauma: it is the cause of children’s legs green and purple, if the children accidentally bumped and bruised, or sprained in sports, the injured parts will be red and swollen, subcutaneous capillary rupture, appearing green and purple. 2. Thrombocytopenia: If the number of platelets in children decreases or their function is low, petechiae and ecchymosis will appear spontaneously on the skin, and even blood in stool and blood in urine, so there will be bruises and purple spots on the legs. 3. allergic purpura: manifested as skin purpura, gastrointestinal mucous membrane bleeding, etc., the rash is densely distributed in the distal lower limbs and ankle joints around the purplish-red dots, more into a piece. There are many reasons for children’s legs to be blue and purple, it is recommended to take children to the hospital to do the relevant examination in a timely manner, to clarify the cause of the disease and to give corresponding treatment.