Fisher-Price 79752 Guide De L'utilisateur page 5

WARNING
NEVER put anything on your vehicle seat (such as a cushion)
to raise the booster 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.
NEVER use the booster car seat after it is involved in a crash.
A crash can cause damage to the booster car seat that you
cannot see.
NEVER allow the shoulder belt to be moved off of your child's
shoulder. It is critical that your child not move the shoulder
belt out of position after you have adjusted it.
Booster Car Seat
3