Jesus...
Ok, first... If they are driving, they should be paying 100% of the cost. Period. No excuses here. If they want to drive, they need to pay for it. By footing the bill, you're teaching them bad lessons and diminishing the impact of the cost of driving.
Second, take the fancy ass car they're driving off of your policy with you and your domestic partner. Put the kids on their own policy. It's legal, and it isolates them from the other vehicles.