Rather than evacuate on foot, I would be looking very hard at bicycles and bike trailers. There are many commercial bike trailers designed for carrying children, dogs and cargo. And I just now saw this trailer which is designed to be an "ambulance" and which could perhaps accomodate your mother.

http://bikecart.pedalpeople.com/ambulance.html

I have a large dog trailer built by a Canadian company named "Croozer" and cargo trailer built by Oregon-based "Burley."

As your children age they will be able to ride their own bikes for some distance, freeing you and your wife up to carry only your mother and cargo.

You can cover so much more ground with a bike than on foot. Here are some other bike trailer options:

for persons with "special needs"
http://blueskycyclecarts.com/BSkyHandicap.htm

trailers to pull adults as well as children
http://www.equinoxtrailers.com/

Casualty transport system
http://www.casualtytransport.com/newpolice.htm