Several radiological indices were introduced to evaluate cast adequacy for paediatric distal forearm fractures: cast, gap, padding, Canterbury (reflecting the cast shape and the amount of padding) and three-point indices, and second metacarpal-radius angle (measuring cast ulnar-moulding). The aim of this study is to define which index is most reliable in assessing cast adequacy and predicting redisplacements. Hundred twenty-four consecutive patients (age 5-18) affected by distal both-bone forearm or radius fractures treated with casting were included. These indices and the displacement angles were calculated on the initial radiograph after reduction. Radiographs at 7 and 30 days were taken to assess if the loss of reduction occurred, and measure the displacement deltas (displacement angle at day 30 - displacement angle at day 0). Student's t-test, Chi-square test and Pearson's correlation were used for the statistical analysis. High padding (P = 0.034), Canterbury (P = 0.002) and Cast (P < 0.001) indices showed an association with redisplacements in distal forearm fractures. Both-bone forearm fractures have a higher risk of loss of reduction than radius fractures [odds ratio (OR = 4.99, 95% confidence interval (CI) = 2.21-11.3, P < 0.001]. A higher displacement delta in antero-posterior (Pearson's r = 0.418, P = 0.037) and lateral (P = 0.045) views for both-bone fractures showed an association with a high gap Index. Regarding radius fractures, a high cast index is associated with a higher displacement delta in antero-posterior (P = 0.035). The three-point index and the second metacarpal-radius angle did not show any association with the redisplacement risk. Cast oval moulding without excessive padding may prevent redisplacements in paediatric distal forearm fractures, while casts ulnar-moulding does not.