You'd have to know how many fatalities were the result of being ejected from the car and how many were the result of being unable to get out due to the seatbelt. Others might be the result of things where a seatbelt was irrelevant (extreme crushing, decapitation, etc.)

It's like helmets for motorcyclists; they don't guarantee you'll survive an accident, but they do help prevent brain damage or catastrophic skull crushing. You're still going to die if your lower half is mangled by an 18-wheeler.

And I think it's even more important for children, whose little bodies are more likely to fly around the car's interior if unrestrained. Or, you know, whose little bodies climbing over the seat into the front might distract the driver and CAUSE a fatal accident. You'd have to dig deeper into the data.