Mine aren’t big, so I don’t know if this is helpful, but we use a harness that has a little clip that clips into the seatbelt. It’s pretty secure, they get a bit of leeway, but it will secure them in an accident enough not to turn into a missile.
safer but lets face it. the moment your taking a motor vehicle you greatly increased your unsafe condition and the restraint is a drop in a bucket at that point..
Modern vehicles are exceptionally safe when operated responsibly. Safety restraints and safety equipment have come a long way, as long as you’re wearing them.
However an unrestrained body in a vehicle becomes a projectile.
In the USA this is one of the reasons why it is easily constitutional to have laws requiring the use of seatbelts, because your unrestrained body isn’t just your problem.
That depends a lot on the vehicle. If you're in something small like a sedan, you might as well be on a bike for all the protection it'll provide from a collision with an SUV or pickup truck