Wiht regard to expense, you wil need to ask your insurer/broker. If they are both over 30 with full licences and no convictions etc, the cost would be substantially cheaper than if they are with provisional licences and young.
the benefit, is that they are insured under your policy!
If they have Driving Other Cars extension under their own policies, then they would probably, (you will need to check their policy wordings) then they can drive your car under their own policy, but on a third party only basis.
If you have comprehensive cover, the only way to have comprehencive cover for them whilst driving your car, is for them to be named drivers.