This is a bit of an unusual review--we had booked a prepaid rental car and, due to our my mistake, failed to bring the ID needed to actually rent the car.
Our family was put in a very difficult situation due to my mistake, and the Sixt staff rightly insisted their policies would not allow us to rent a car. The Sixt staff, however, were unfailingly kind, asked all the other rental car companies whether their policies would allow us to rent a car, gave us advice on finding a car service, and even loaned us a baby seat for my family to use despite the fact that we couldn't rent from Sixt.
My family felt well-cared for despite a difficult situation and give many thanks for the humane, caring staff at the Marseille Airport Sixt.