If your son, daughter, wife, husband or any other policyholder on your cover makes a claim, or a claim is made against them, while they're driving their own car it won't affect any of the other policyholders' No Claims Bonus. However, if they have an accident in your car (which you are policyholder for) then your No Claims Bonus will be affected, but not for the other cars.
Widespread use of the motor car began after the First World War in urban areas. Cars were relatively fast and dangerous by that stage, yet there was still no compulsory form of car insurance anywhere in the world. This meant that injured victims would seldom get any compensation in an accident, and drivers often faced considerable costs for damage to their car and property.