A 2-year-old child presents with a barking cough, hoarseness, and inspiratory stridor, which started suddenly overnight. Symptoms are worse when upset. The child is febrile (38.2°C) but appears generally well and playful between coughing episodes, with no signs of significant respiratory distress at rest. Oxygen saturation is 97% on air. What is the most likely diagnosis and appropriate initial management according to NICE?