A child with a rash on the legs and swollen knees may have insect-bite dermatitis, allergic purpura, and should seek medical attention to clarify the cause of the disease and then treat it. Insect bite dermatitis: If a child has a possible mosquito bite before the occurrence of a rash on the legs swollen knees, insect bite dermatitis should be excluded. In addition to the leg rash and localized swelling, the patient will have conscious itching and burning pain, which can be treated with oral loratadine. Anaphylactic purpura: a common allergic reaction to inflammation, often manifested as a symmetrical red rash on both legs, may also be accompanied by arthralgia, low-grade fever, loss of appetite, malaise, etc., the acute attack patients should be absolutely bed-rest, can be used naproxen, methylprednisolone and other drugs for treatment. Children’s leg rash knee swelling may also have other reasons, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.