Generally speaking, the causes of constant bruising on the lower legs of children are due to the following two diseases. The first is caused by idiopathic thrombocytopenic purpura. If the child is young, between one and three years old, this is when the child tends to suffer from idiopathic thrombocytopenic purpura. When a child’s platelets are tested, the child’s platelet count is often very low, because platelets are very useful for the child’s blood clotting function, and when the child’s platelets are lowered, the child’s blood clotting function decreases, which can cause the child to have bruising. For children with thrombocytopenic purpura, parents need to apply glucocorticoids or anti-infection treatment to their children in time. The next is caused by allergic purpura, if the child suffers from allergic purpura, there will also be capillary bleeding in the legs, the child’s calves will appear to have bruises, for this situation, parents need to give the child timely anti-allergy treatment.