Background/Objectives: Transcatheter closure is the treatment of choice for hemodynamically significant secundum atrial septal defects in anatomically suitable children. However, procedural risk assessment remains challenging in smaller patients. We assessed safety and effectiveness across weight groups and explored the contribution of defect size and diameter-to-weight ratio (DWR) to procedural complexity. Methods: We retrospectively analyzed 275 children who underwent transcatheter closure of isolated secundum atrial septal defect between 2014 and 2024 at a tertiary pediatric cardiac center. Patients were stratified according to body weight: ≤20 kg and >20 kg. Clinical, procedural, and echocardiographic data were reviewed, including defect size, device type, rim adequacy, and diameter-to-weight ratio. Septal rim characteristics were descriptively assessed in patients experiencing device embolization. Results: Procedural success was achieved in 270/275 patients (98.2%). Eleven adverse events occurred (4.0%), with no significant difference between weight groups (2.9% vs. 5.0%; p = 0.54). Body weight did not differ significantly between patients with and without intraprocedural (p = 0.18) or postprocedural (p = 0.71) complications. DWR was higher in patients weighing ≤20 kg. Five patients experienced device embolization (1.8%); all had unfavorable septal rim characteristics on retrospective review. No device erosion, cardiac tamponade, major vascular complications, pacemaker implantation, or long-term sequelae occurred. Conclusions: Transcatheter atrial septal defect closure can be performed safely and effectively in appropriately selected children, including those weighing ≤20 kg. No significant association between body weight and complications was demonstrated, although few events limit inference. The predictive value of DWR and rim adequacy requires further study.