I always travel with water and a portable toilet I think I'm going to add a blanket as well.....
I have a xsara picasso and they do have some extra spaces bellow the floor pannels in the back passengers side. It's a good space to store this kind of things
"Best case scenario we have extra food, worse case scenario we have food"
We have always kept sleeping bags or blankets in the car. Some years ago we had to evacuate our flat at night after a neighbour went up in flames. Our kids and our neighbours went in the bags in the car while the adults waited to find out if/when we would be allowed back home. Kids all safe, secure and warm despite the chaos.