I just want to underline what cheapChic said: an extra pair of sturdy, comfortable shoes for every family member and rain clothes.
That goes in under plan B too: prepare to walk or use alternative transportation than you are used for.
A thin rope is also good to have and a few plastic sheets or the like. Can be used for anything from making a shelter, if you don't have (time to put up) a tent, to gathering water.
Iodine solution for water purification is very good. Doesn't take as much place as chlorine bleach and you can use less of it too!
Great job, cheapChic! Just want to agree with you!