SeatSnug improves the safety and comfort of children riding in belt positioning car booster seats. Children in booster seats are at risk of injury due to seatbelt slack (looseness). The three-point seatbelt commonly used in motor vehicles represents a compromise between safety and comfort. A three-point seatbelt allows slack to develop in the lap belt portion while children are riding in a vehicle. The SeatSnug allows the lap belt to be lightly tightened around a child's hips while riding in a vehicle, preventing seatbelt slack or looseness from developing. By gently securing a child while riding in a booster seat, SeatSnug substantially eliminates bouncing, rocking, tipping, falling over, and submarining. SeatSnug can be easily and quickly clipped onto a vehicle's existing factory installed seatbelt. Once a child in a booster seat is buckled up, the lap belt may be lightly tightened around the child's hips by pulling up on the shoulder strap to more safely secure the child, without affecting the normal operation of the shoulder strap.