GMC 2005 Envoy Owner's Manual page 42

The restraint manufacturer's instructions that come
with the restraint state the weight and height
limitations for a particular child restraint. In addition,
there are many kinds of restraints available for
children with special needs.
CAUTION:
{
Newborn infants need complete support,
including support for the head and neck.
This is necessary because a newborn infant's
neck is weak and its head weighs so much
compared with the rest of its body. In a crash,
an infant in a rear-facing seat settles into
the restraint, so the crash forces can be
distributed across the strongest part of an
infant's body, the back and shoulders. Infants
always should be secured in appropriate
infant restraints.
1-36
CAUTION:
{
The body structure of a young child is quite
unlike that of an adult or older child, for whom
the safety belts are designed. A young child's
hip bones are still so small that the vehicle's
regular safety belt may not remain low on the
hip bones, as it should. Instead, it may settle
up around the child's abdomen. In a crash, the
belt would apply force on a body area that is
unprotected by any bony structure. This alone
could cause serious or fatal injuries. Young
children always should be secured in
appropriate child restraints.