Purpose This study aimed to investigate the gross motor functions including head control, midline crossing, and rolling, and the relationship between these developmental skills and upper extremity skill quality in children with neonatal brachial plexus palsy (NBPP). Methods A total of 106 children with NBPP, aged 10-18 months, were included in this study. Injury severity was determined with the Narakas Classification. The gross motor function measurement lying and rolling sub-scale was used to evaluate gross motor functions including head control, midline crossing, and rolling, while the Quality of Upper Extremity Skills Test was applied to assess the upper extremity skill quality. The assessments were performed only once during routine physiotherapy controls. Results As the severity of injury increased, developmental skill capacity decreased and upper extremity skill quality deteriorated (ps = 0.0001). There was a strong positive correlation between these developmental skills and upper extremity skill quality (ps = 0.0001). Conclusion Developmental skills are affected by NBPP. Rehabilitation programs aimed at increasing the quality of upper extremity skills should be included in neurodevelopmental treatment approaches.