I agree with everything that's been said here. When my son was young, I did not start the engine until all seat belts were fastened.

That being said, I grew up riding without seat belts or car seats. One suggestion: If someone absolutely refuses to buy a car seat or doesn't have one, see if you can talk them into placing the child in a well-padded laundry basket in the floor of the car and take a kitchen towell (what we call a tea towell) and tie it across the middle of the basket. The more padding you can put around the child and immobilize them, the less likely they are to bump against something and get hurt. Also, place padding around their head to keep it from going side to side.

I'm not sure about older children. Perhaps, padding them with blankets to make them more secure?