Howdy Michelle and everyone else,
This is a practical and excellent idea, combining root cellar and shelter for oh, what brouhaha may come and of course those things of a more serious nature. To that end, I would recommend the book, Nuclear War Survival Skills by Cresson H. Kearney. It is detailed in such a way that you find in books of yore. In addition, as far as root cellaring is concerned I have and would recommend Root Cellaring by Mike and Nancy Bubel.
On another note, if memory serves, Nader Khalili was the originator if you will, of earth bag or super adobe etc. construction, which others have written about. This vernacular originates from arid areas and therefore, at least to my mind, would give one pause in understanding how it would translate to one's place. In your case, the foothills of Seattle. I'm of the opinion that PSP, post shore polyethylene, construction by Mike Oehler would be worth looking into. You can find some info about that here on Permies. Last, I like your tag line, "Just a mom, trying to do best by her kids." Good on you! and good luck on your project!