Water. Then some water. Get a little more water. See the trend? You have got to have water for hydration, sanitation, cooking, first-aid, and taking care of 4-legged friends. There are countless other things you need it for but if you are without resources for any length of time you cannot have too much of it. You are smart people (I hope)and realize that the bigger the family the more you might need. Laying in a huge stock of bottled water can take up some room but it is worth it and you can use it up during times of the year when you don't necessarily need to keep it on hand. My basic formula for how much is to contact your local emergency preparedness office and find out what they recomend per person per day then double that. Then figure out what the longest time you would expect to be stranded is and double that. then multiply those 2 figures coming up with a formula like this where amount=A time=T and water equals=W
2A X 2T = W ex: 3 gallons per person per day and you estimate 7 days. So useing my formula it would be 6gal X 14Days =84 gal per person for that period. This can be adjusted for peoples size and activity level meaning that capable adults involved in cleanup and housekeeping can be expected to require more per day than elderly and children. The time you plan for is also adjustable by location. If you live in an urban area or even suburban area with decent access and high ground you can scale it back. If you live out in the woods without any access if your roads are blocked you may want more.