We evaluated the morphological changes to the ulnar nerve of both elbows in the cubital tunnel by sonography in a total of 237 children, of whom 117 were aged between six and seven years, 66 between eight and nine years, and 54 between ten and 11 years. We first scanned longitudinally in the extended elbow and then transversely at the medial epicondyle with the elbow extended to 0°. We repeated the scans with the elbow flexed at 45°, 90°, and 120°. There were no significant differences in the area of the ulnar nerve, but the diameter increased as the elbow moved from extension to flexion in all groups. More importantly, the ulnar nerve was subluxated anteriorly on to the medial epicondyle by 1.5% to 1.9% in extended elbows, by 5.9% to 7.9% in those flexed to 45°, by 40.0% to 44% in those flexed to 90°, and by 57.4% to 58.1% in those flexed to 120°, depending on the age group. Sonography clearly and accurately showed the ulnar nerve and was useful for localising the nerve before placing a medial pin. Because the ulnar nerve may translate anteriorly onto the medial epicondyle when the elbow is flexed to 90° or more, it should never be overlooked during percutaneous medial pinning.