A 3-year-old boy presents to the emergency department with a 2-day history of low-grade fever, rhinorrhea, and cough. Over the past 6 hours, he has developed a characteristic barky, seal-like cough and mild stridor at rest. His mother reports he was well until 48 hours ago when he developed upper respiratory symptoms. On examination, he is alert, maintaining oxygen saturation of 96% on room air, with a respiratory rate of 28/min. Auscultation reveals decreased air entry bilaterally with scattered rhonchi. There is no drooling or tripoding. What is the most appropriate initial management?