Irresponsible parents are kid killers. During nearly twenty years as a professional firefighter and paramedic, I've responded to many automobile accidents involving child fatalities. However during that time, I've never had to remove a dead child from a rear seat that was wearing a properly worn seat belt.
No, a seatbelt is not 100% insurance that anyone will survive an automobile accident but properly worn seat belts increase the odds of surviving dramatically for both children and adults.