What is the cause of children's elbow bone protrusion?

Hello, generally speaking, you don't think this is a disease. There are bony processes in human elbow joint, which are called lateral epicondyle and medial epicondyle.

If you are worried about the problem, you can go to the hospital for an X-ray examination, which will be clear at a glance. But I don't think you need to worry too much. This is a normal bony process. I wish you good health.