Abstract: A child's rear-facing safety seat is secured in a substantially horizontal position on a vehicle rear seat that has a seat surface extending downwardly to a joinder with a seat back, by positioning an apex of a bolster having the shape of an elongated, triangular wedge into the seat joinder, and placing the child's seat in a rear-facing position on the seating surface with the front edge of the child's seat resting on the bolster, so that the seat is maintained by the bolster in a substantially horizontal position. The bolster is preferably formed of a central section in the shape of a equilateral triangle with opposed first and second parallel, rectangular surfaces; and first and second triangular side sections, each having a rectangular side surface releasably attached to a rectangular surface of said central section. When placing a child's seat on a vehicle rear seat that has a smaller tilt, the central section can be removed, and the bolster formed by joining the side sections to each other.