I let my 5 and 3 year old son ride in the car, but would probably not allow a newborn to ride in there as they are very fragile. That being said, there are other ways to work around this issue, they can bring your grand daughter over when they want you to
babysit or your can swap cars. I am sure they will be more than happy to work with you as you are the most trustworthy person they can leave...