The common clinical causes of leg pain in 2-year-old babies include growing pains, synovitis, osteosarcoma, and trauma. 1. Growing pains: 2-year-old babies are in a period of growth and development, bone and muscle growth is not coordinated, resulting in growing pains. 2. Synovitis: this condition is usually due to the local joints frequent activities or improper activities caused by a kind of synovial inflammatory changes, clinical manifestations of local joint swelling, pain, and joint activities are limited. 3. Osteosarcoma: this is a kind of malignant tumor, mostly occurring around the knee joints, mostly seen in adolescents and children’s groups, with clinical manifestations such as pain and swelling of the local tissues around the joints, as well as limitation of joint activities. 4. Trauma: when the baby’s leg is hit by external force, it will cause local tissue pressure injury, abrasion, etc., with clinical manifestations of pain, swelling or bleeding in the affected area. If there is a relevant situation, parents are advised to be vigilant and go to the hospital in time to clarify the cause.