Edward,
You can start by searching for online car insurance quotes to see how much you're likely to pay for the premiums with 6 named drivers and you as the policyholder. If you notice that the quotes are you getting are too high, then maybe you should consider whether it's worth having all of your kids as named drivers on the policy. If they are going to use the car regularly, then that's fair enough and they will have to go on the same policy. Mind you though that a car shared across 7 drivers is going to be difficult as arguments will arise on who gets the car when and for how long.
Instead of putting them all as named drivers on one car insurance policy, you can also add only those kids who will use the car more often. The rest can be insured through
short term car insurance when they need to drive it (age restrictions may apply though).