I don't think it is a good idea to take the kids in your car for the reasons cited above. If I must, like when I take an older student with me to assist me when I guest teach at other studios and seminars or conventions, I have a waiver signed by the parents and carry all of their emergency medical info in case of an emergency.

Tell them you are no longer allowed to carry the kids in your car for insurance purposes and learn how to say no.