Children assume a range of postures when utilising belt-positioning boosters, which may influence belt fit and have implications for dynamic performance. This study evaluates the belt fit and posture of children on boosters while assuming different postures:​ self-selected, holding device, and nominal. Children (n=25) were recruited (4–11 years, 103.0–146.5 cm, 17.8–33.6 kg) and evaluated on two of five randomised boosters. A 3D coordinate measurement device and an inertial measurement unit-based motion capture system quantified posture, e.g., head, torso, and pelvis positions and orientations, and belt fit, e.g., shoulder belt score, lap belt score, maximum gap size, gap length. Outcomes were compared across postural conditions and boosters using repeated-measures ANOVA. The device condition produced significantly more forward and flexed head postures compared to self-selected and nominal (by 58 mm and 15.0° on average). Variation was small in terms of belt fit and belt gap metrics between postural conditions, suggesting that belt routing features provided similar belt placement despite postural adjustment;​ however, greater variation is expected in naturalistic settings. This study is the first to directly evaluate the posture and belt fit of children while holding electronic devices and to investigate the influence of different postural conditions on belt fit and postural outcomes.