" My inlaws 'get' car seats for babies but I know they have taken my (older than my DCs) nephews in their car without booster seats."
It's a matter of degree, though, isn't it? Small children without seats: massively increased risks in pretty well any accident or even emergency braking situation.
Larger children (say 5/6) without booster seats: slightly increased risk of secondary injuries in high-energy accidents.
There's no such thing as complete safety in cars: given enough energy, and enough bad luck, you will die whatever precautions are in place (as Ayrton Senna's death shows). And sadly, the best things you could do to improve passive safety aren't acceptable for general road users.
The first is helmets, because head injuries in cars are the main killer. Paradoxically, more lives would be saved if people in cars wore open-face helmets, rally-style, than if all cyclists wore helmets. The increase in neck injuries (HANS devices and head tethers aren't acceptable in road cars) would have to be borne in mind, but the energy involved in a typical road accident is such that a normal open-face helmet probably isn't going to impose a huge extra loading. The second is four-, five- or six-point harnesses, because three-point seatbelts are shit. But crotch straps won't be acceptable because of people wearing skirts and dresses, and it's very difficult to rig the "inside" shoulder strap in a four seat car without massive changes to seats. Car seats for children typically have five point harnesses, which is good, but it's all a safety/usability trade-off.
Overall risk exposure scales (roughly) with how far you drive and maximum speed, so fifty miles on mixed roads with your child properly secured in the correct seat is probably still a great deal more risky than three miles without correct seats never getting out of third gear. On the one hand, you can argue about reducing unnecessary risk, but then you would focus on not using the car in the first place. On the other hand, once you've taken the risk of using the car at all, it's likely that small differences in passive safety (booster seats) don't significantly alter your chances of death or serious injury, while other differences (using or not using a baby seat) are still very important.