This study evaluated serum eosinophil cationic protein (ECP) as a biomarker for pediatric allergic airway diseases. A cross-sectional analysis was performed on children (1–17 years) with allergic asthma (AA, n = 124), allergic rhinitis (AR, n = 74), acute bronchitis (AB, n = 72), and healthy controls (HC, n = 58). Serum ECP, total IgE, eosinophil counts, allergen sensitization, and lung function were measured. Diagnostic performance was assessed using receiver operating characteristic (ROC) curves, and correlations among biomarkers were examined. Compared with HC, serum ECP levels were significantly elevated across all disease groups (AA, AR, and AB), with a particularly marked difference observed between AA and AR patients (p < 0.0001). The combination of ECP and IgE significantly improved the diagnostic accuracy for AA (AUC = 0.9494) and AR (AUC = 0.9501). Higher ECP levels were associated with increased sensitization to specific inhalant allergens and impaired pulmonary function, particularly in small airway indices. Serum ECP reflects eosinophil-mediated airway inflammation and enhances diagnostic performance for pediatric AA and AR, supporting its role as an auxiliary biomarker in evaluating pediatric allergic airway diseases.