The "Be an Airplane Pilot" (BE-API) protocol is a novel 3-D movement analysis (3DMA) protocol assessing the bimanual performance of children during a game. This study aimed to investigate the reliability and validity of this protocol in children with unilateral cerebral palsy (uCP). Angular waveforms (WAVE), maximum angles (MAX) and range of motion (ROM) of the trunk, shoulder, elbow and wrist joints were collected in children with uCP and in typically developing children (TDC) during 4 tasks of the BE-API protocol designed to explore specific degrees of freedom (DoF). The inter-trial reliability was assessed with the coefficient of multiple correlation (CMC) for WAVE and the intraclass correlation coefficient (ICC) and standard error of measurement (SEM) for MAX and ROM. Clinical performance-based measures, including the Assisting Hand Assessment (AHA) and ABILHAND-Kids scores, were used to explore correlations between clinical measures and kinematic parameters in children with uCP. For the 20 children with uCP (13 boys; mean age 12.0 [SD 3.2] years) and 20 TDC (11 boys; mean age 11.9 [SD 3.4] years), most kinematic parameters showed high reliability (WAVE: CMC≥0.82; MAX and ROM: ICC≥0.85, SEM≤4.7°). Elbow extension, forearm supination, and wrist adduction were reduced and wrist flexion was increased for children with uCP versus TDC (P<0.01). In children with uCP, MAX and ROM values were moderately correlated with clinical assessments (AHA score: r=0.48-0.65; ABILHAND-Kids score: r=0.48-0.49). The BE-API protocol is a 3DMA-bimanual performance-based assessment that is highly reliable in children with uCP. Children with uCP and TDC significantly differed in some clinically relevant kinematic parameters. The BE-API is a promising playful tool, helpful for better understanding upper-limb motor movement abnormalities in bimanual conditions and for tailoring treatments to individual deficits.