Traumatic ulcer: This is the most likely diagnosis given the acute onset and the fact that the child denies any systemic symptoms like fever. Traumatic ulcers can occur from accidental bites, eating hot foods, or other minor injuries to the tongue.

Herpes simplex virus (HSV) infection: Although the child denies fever, HSV can cause oral ulcers without systemic symptoms, especially in children. The presence of a single sore could be an initial presentation.
Aphthous ulcers: These are common in children and can appear as a single sore on the tongue. They are usually self-limiting and can be triggered by minor trauma or stress.
Oral candidiasis: This fungal infection can cause white patches or red lesions on the tongue, but it's less likely if the child is otherwise healthy and doesn't have a history of immunocompromised conditions or recent antibiotic use.

Squamous cell carcinoma: Although extremely rare in children, any persistent or non-healing ulcer should raise suspicion for malignancy. Early detection is crucial.
Infectious mononucleosis (mono): Caused by Epstein-Barr virus (EBV), it can present with oral ulcers, although typically accompanied by fever and lymphadenopathy.
Hand, foot, and mouth disease (HFMD): Usually presents with fever and a rash on the hands and feet, but oral ulcers are a key feature. The absence of fever doesn't rule it out entirely.

Erythema multiforme: An immune-mediated condition that can cause oral ulcers, but it's usually associated with other mucocutaneous lesions and systemic symptoms.
Autoimmune disorders (e.g., pemphigus, pemphigoid): These conditions are rare in children and typically present with more widespread oral and skin lesions.
Leukemia: Certain types of leukemia can cause oral ulcers due to thrombocytopenia or immune suppression, but this would be accompanied by other significant symptoms such as weight loss, fatigue, and petechiae.