Buick 2003 Rendezvous Owner's Manual page 58

Never do this.
Here a child is sitting in a seat that has a
lap-shoulder belt, but the shoulder part is
behind the child. If the child wears the belt in
this way, in a crash the child might slide under
the belt. The belt's force would then be applied
right on the child's abdomen. That could cause
serious or fatal injuries.
The lap portion of the belt should be worn low and snug
on the hips, just touching the child's thighs. This
applies belt force to the child's pelvic bones in a crash.
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