Large numbers of children have been admitted to the Children's National Medical Center in Washington, DC for a complex of injuries caused by lap styled safety belts.
The Departments of Pediatrics and Diagnostic Imaging and Radiology reported that 61 children who had been restrained passengers in car crashes had linear ecchymosis (doctorese for "heavy bruising") across the abdomen and abdominal trauma.
X-rays showed lumbar spinal injury in all cases.
It may well be that shoulder harnesses alone are safer designs for children. If in doubt about your particular brand of child car seat, phone the Consumer's Association for the result of any safety tests.