Musculoskeletal injuries are a frequent cause of paediatric injuries and emergency department visits in Western countries. Diagnosis typically involves radiography, but this exposes children without fractures to unnecessary ionising radiation. This study explored whether infrared thermography could provide a viable alternative in trauma cases. Radiography and thermal images of 133 children who had been diagnosed with a trauma injury in the emergency unit of a Spanish hospital were compared. As well as the thermal variables in the literature, a new quantifier variable, the size of the lesion, is introduced. Decision tree models were built to assess the technique’s accuracy in diagnosing whether a bone had been fractured or not. Infrared thermography had a sensitivity of 0.91, a specificity of 0.88 and a negative predictive value of 0.95. The new lesion size variable introduced seemed to be of main importance to the discriminatory power of the method.