I do not have an authoritative answer to give you regarding their surrendering half of their savings to put toward these increased insurance costs. How did you arrive at their giving you half of their savings? Can they contribute a percentage of their summer job earnings, as opposed to taking it out of their savings? The debate that you will have with them is not whether they should contribute, but how much they should contribute.
Having a car is a privilege, not a right. I hope you can all arrive at a fair contribution on their part to help with the insurance costs and the maintenance of these cars. I trust that you will be fair in your requests and that they will understand their responsibilities in sharing these costs. You all may come to the conclusion that it is too expensive for the family to maintain these two additional cars. They are certainly not necessities. They are luxuries. This reality should inform your discussions and negotiations.