Fisher-Price GROW-WITH-ME 79711 - Manual (Page 78)

Restraining Your Child
WARNING
NEVER put anything on your vehicle seat (such as a cushion)
to raise the car seat. This will create too much slack in the
vehicle belt system. In a sudden stop or crash, a loose vehicle
belt system will not be able to properly restrain your child.
Booster Car Seat Mode 18 30-80 lbs.