Motor vehicle collisions remain one of the leading causes of death for children despite the high effectiveness of child restraint systems. Specifically, belt-positioning booster (BPB) seats elevate children between 4 and 12 years old to achieve appropriate belt positioning. However, occupant posture also contributes to belt fit. Previous work has shown that BPB occupants’ postures, and subsequently safety, are highly dependent on their environment and comfort. There are limited investigations into the varying nature of BPB occupant posture over extended time periods and the relationship to comfort motivated factors. Pediatric volunteers (n=30) were randomly assigned to two out of five possible seating configurations installed in a mock vehicle setting. Ideal reference postures were recorded, then volunteers were instructed to assume comfortable postures throughout a 30-minute trial during which postural data were continuously collected from XSENS inertial measurement units and video footage. Children significantly changed their posture over time trending towards slouched postures. These changes in posture were greatest for the no-BPB configurations. Incompatibility between child anthropometry and seating configurations contributes to such behaviors as children accommodate to the seat geometry to become comfortable. Future work can utilise these data to evaluate injury risks associated with naturalistic postures.