Background: Despite medical advances, individuals with cerebral palsy (CP) face significant respiratory challenges, leading to heightened hospitalization rates and early mortality among this population. We hypothesize that integrating supplementary respiratory therapy into standard rehabilitation will result in significant improvements in pulmonary function, enhanced respiratory muscle strength, and an overall increase in the quality of life among pediatric patients with CP. Methods: A systematic search of literature across five databases was conducted, and random-effects meta-analyses were performed to assess the impact of supplementary respiratory therapy on (a) pulmonary function: forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), FVC/FEV1 ratio, peak expiratory flow (PEF), and (b) respiratory muscle strength: maximal inspiratory and expiratory pressure (MIP, MEP), and (c) quality of life. Certainty of evidence was determined by the GRADE assessment. Results: Analysis of data from 11 eligible randomized controlled trials revealed clinically meaningful changes in pulmonary function. We found a relevant mean difference (MD) in absolute PEF of 0.50 L/s (95% confidence interval (CI): 0.19; 0.82 p = 0.0107). The certainty of the evidence ranged from moderate to high. Conclusions: This study presents current evidence on the impact of various supplementary respiratory therapies for CP patients classified under gross motor function classification level I–IV, demonstrating clinically meaningful improvements in pulmonary function and respiratory muscle strength. These improvements suggest the potential for an enhanced quality of life. Our findings hold the promise of serving as a foundational reference for potential revisions to conventional rehabilitation care, incorporating supplementary respiratory therapy.