Chevrolet 2010 Impala Owner's Manual page 64

WARNING:
{
Never do this.
Never allow a child to wear the safety belt with the
shoulder belt behind their back. A child can be
seriously injured by not wearing the lap-shoulder
belt properly. In a crash, the child would not be
restrained by the shoulder belt. The child could
move too far forward increasing the chance of
head and neck injury. The child might also slide
under the lap belt. The belt force would then be
applied right on the abdomen. That could cause
serious or fatal injuries. The shoulder belt should
go over the shoulder and across the chest.
2-34