Dan,
Unfortunately, there are no car insurance companies on the market at the moment which will insure drivers under the age of 21. This is probably because younger drivers are at a higher risks and giving them insurance cover will more likely result into claims which insurers don't really want as this will bring down their profits.
If they are
learner drivers, then you can get short term insurance for them. Otherwise they will need to be added as named drivers on somebody's else policy.
