Precordial Catch Syndrome: This condition is characterized by sharp, stabbing chest pain that can feel like a punch to the chest. It is often brief, lasting seconds to minutes, and can occur several times a day. The pain is typically nonpleuritic and can be triggered by specific movements or positions. Given the description of the pain and the absence of other symptoms, this diagnosis fits well with the clinical presentation.

Costochondritis: This is an inflammation of the cartilage that connects the ribs to the breastbone. It can cause sharp pains in the chest that may worsen with movement or deep breathing. Although the pain described is nonpleuritic, costochondritis could still be a consideration, especially if there are tender points on the chest wall.
Stress or Anxiety-Related Chest Pain: Children, especially preteens, can experience chest pain related to stress or anxiety. The pain can be sharp and fleeting, similar to what is described. Given the absence of other physical findings and the potential for psychological stressors, this is a plausible diagnosis.
Gastroesophageal Reflux Disease (GERD): Although the question states no reflux, GERD can sometimes present with atypical symptoms, including chest pain that is not clearly related to eating or positional changes. However, the lack of other symptoms like heartburn or abdominal discomfort makes this less likely.

Myocarditis or Pericarditis: These are inflammatory conditions of the heart that can present with chest pain. Myocarditis might also explain the mild bradycardia. Although less common in children without a clear infectious or autoimmune trigger, these conditions are critical to rule out due to their potential severity.
Pulmonary Embolism: Extremely rare in children without underlying risk factors (e.g., thrombophilia, recent surgery, immobilization), but it is a diagnosis that could be deadly if missed. The presentation would typically include more systemic symptoms, but it's essential to consider in any differential for chest pain.
Cardiac Channelopathies (e.g., Long QT Syndrome): These conditions can cause arrhythmias and might present with chest pain, although they are more commonly associated with syncope or palpitations. The mild bradycardia noted could be a red herring, but any abnormal heart rhythm warrants further investigation.

Mitral Valve Prolapse: This condition can cause chest pain, but it's more commonly associated with palpitations, murmurs, or other cardiac findings. It's less likely given the normal heart exam aside from mild bradycardia.
Spontaneous Pneumothorax: Extremely rare in healthy children without underlying lung disease, but it would present with acute, severe chest pain and possibly shortness of breath, which does not match this clinical scenario.
Tietze's Syndrome: Similar to costochondritis but involves the inflammation of the cartilages and surrounding tissues of the ribs. It's less common and typically presents with swelling and tenderness of the chest wall, which is not described in this case.