Phillco, my condolences on your mother's passing.
What are the ages of the grandchildren? If there are young ones (teenagers or younger), it might be too stressful for them to be alone in their own car. Maybe you and your siblings in the first car, and spouses and grandchildren in the second. Young ones shouldn't be without an adult handy in such a stressful time.
(I'm assuming, of course, that spouses wouldn't feel slighted at not riding in the first car. I've been to a fair number of funerals for my husband's family, and I'm amazed at how political these can get sometime.)
Your funeral director should have standard recommendations - and if the seating order comes from him/her, there's less a chance that someone will feel slighted. Best wishes to you and your family.