Red spots on the back of a child’s knee may be related to eczema, contact dermatitis, thrombocytopenic purpura and other causes. 1. Eczema: If eczema appears behind the child’s knees, it can be manifested as a papule on top of the red spot. 2. contact dermatitis: if the skin behind the child’s knee is exposed to allergenic substances, it can cause an abnormal immune response, manifested as red spots on the skin. 3. Thrombocytopenic purpura: If you suffer from thrombocytopenic purpura, you may experience capillary rupture and hemorrhage behind the knee, which is manifested as red dots behind the knee. It is recommended that parents take their children to the hospital in time to clarify the cause of the disease and standardize the treatment under the guidance of the doctor, so as not to delay the condition.