Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of sudden cough in children, often presenting with other symptoms like runny nose, sore throat, and fever. The high frequency of viral infections in this age group makes it the most likely diagnosis.

Asthma: A common condition in children that can cause cough, especially if triggered by respiratory infections, allergens, or irritants. A history of wheezing, shortness of breath, or previous asthma diagnoses would support this diagnosis.
Allergic Rhinitis: Allergies can cause cough due to postnasal drip. Seasonal or perennial allergies could be the culprit, especially if the child has other symptoms like sneezing, itchy eyes, or a runny nose.
Pertussis (Whooping Cough): Although vaccination has reduced its incidence, pertussis can still occur, especially in areas with low vaccination rates. The characteristic whoop sound during coughing is a key diagnostic feature.

Pneumonia: Bacterial pneumonia can present with sudden onset of cough, fever, and possibly difficulty breathing. It's crucial to consider this diagnosis due to the potential severity and need for antibiotic treatment.
Foreign Body Aspiration: This is an emergency that can cause sudden onset of cough, choking, or difficulty breathing. It's more common in younger children but can occur in older children as well.
Inhalation Injury or Exposure: Exposure to smoke, chemicals, or other irritants can cause a sudden cough. This could be due to accidental inhalation or environmental exposure.

Cystic Fibrosis: A genetic disorder that can cause chronic cough among other respiratory and gastrointestinal symptoms. It's less likely to present for the first time with a sudden cough in a previously healthy 10-year-old but should be considered in the differential for chronic or recurrent cough.
Tuberculosis (TB): Although rare in many parts of the world, TB can cause cough and is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.
Bronchiectasis: A condition characterized by damaged airways, leading to chronic cough and recurrent infections. It could be a consideration in a child with a history of recurrent respiratory infections or underlying conditions that predispose to bronchiectasis.