The forearm function in children is influenced by several factors, with the length relationship between the ulna and radius being particularly important. However, due to the underdevelopment of cartilage in children, assessing this length relationship through imaging techniques is challenging. This study aims to identify a reliable method for evaluating the lengths of the ulna and radius in children, establish normal values for the ratio of the epiphyseal plate lengths of these bones, examine this ratio in children with Monteggia fractures, and investigate its role in determining forearm function in the pediatric population. This multicenter study involved 432 normal children under 15 years of age as controls. A retrospective analysis was conducted on anteroposterior and lateral radiographs from 58 children with Monteggia fractures after surgery. The correlation between the inner-epiphysis ulna-radius length ratio, postoperative Mayo elbow joint function score, and radiographic reduction quality of the brachioradial joint was analyzed. The mean inner-epiphysis ulna-radius length ratio was 1.094, with a standard deviation of 0.024. The maximum value ranged from 1.0336 to 1.168. The medical reference range, with a 95% confidence level, was calculated as 1.047 to 1.141. In children with Monteggia fractures, the postoperative inner-epiphysis ulna-radius length ratio showed significant correlations (P <
0.05) with postoperative imaging reduction quality, as well as the range of pronation, supination, flexion, and extension, and the Mayo elbow score. In normal children, the inner-epiphysis ulna-radius length ratio ranges from 1.0336 to 1.168, with a medical reference range of 1.047 to 1.141 at a 95% confidence level. Restoring this ratio (1.047-1.141) in children with Monteggia fractures can improve elbow function and radiographic radiocapitellar joint reduction quality.