Forearm fractures in children have a tendency to displace in a cast leading to malunion with reduced functional and cosmetic results. In order to identify risk factors for displacement, a total of 247 conservatively treated fractures of the forearm in 246 children with a mean age of 7.3 years (sd 3.2; 0.9 to 14.9) were included in a prospective multicentre study. Multivariate logistic regression analyses were performed to assess risk factors for displacement of reduced or non-reduced fractures in the cast. Displacement occurred in 73 patients (29.6%), of which 65 (89.0%) were in above-elbow casts. The mean time between the injury and displacement was 22.7 days (0 to 59). The independent factors found to significantly increase the risk of displacement were a fracture of the non-dominant arm (p = 0.024), a complete fracture (p = 0.040), a fracture with translation of the ulna on lateral radiographs (p = 0.014) and shortening of the fracture (p = 0.019). Fractures of both forearm bones in children have a strong tendency to displace even in an above-elbow cast. Severe fractures of the non-dominant arm are at highest risk for displacement. Radiographs at set times during treatment might identify early displacement, which should be treated before malunion occurs, especially in older children with less potential for remodelling.