I think the next important thing besides food and water is an emergency blanket. You can find them pretty cheap, about $1.25, for each member of your family; add some to your car as well if you don't have something already.
Read more info on blankets here. I have a bungy cord hooked to my kids backpacks and if time permitted they would be able to roll up their "blankies" and carry it that way. They have the emergency foil blanket in their pack but I felt that they would need that security and warmth of a real blanket, and this is the only way I could fit it. I also carry light weight blankets in my trunk at all times, but they are not the special ones they love:) BUT they would be available if we were not able to grab their own blankets in time.