Submandibular trauma is rare, particularly in children or with an intact mandible. Clinical assessment of a child with a neck mass may need to be supplemented by diagnostic imaging. The optimal management of submandibular abscess is unclear, but an initially conservative approach is recommended. We report a case of a 10-year-old girl with a blunt injury 2 days following a fall who presented with a fever and a large lateral neck swelling. Inflammatory markers were raised and antibiotics were commenced. A magnetic resonance imaging (MRI) scan led to an initial working diagnosis of a submandibular abscess. Over time she remained systemically well, and an ultrasound could not radiologically differentiate infection from a collection. The final diagnosis was a submandibular post-traumatic hematoma with a secondarily infected collection. The initial management and decision to transfer to a higher level of service was significantly aided by the availability of MRI scanning. This case report illustrates the diagnostic and therapeutic dilemma in evaluating and treating acute mandibular swelling following blunt trauma.