The corneredcat.com has already been mentioned enough times; as has training. After that, I would start with a good DOG, something the kids can have fun with and a breed that is normally protective. The dog will be her first lone of defense against an intruder sneaking around.
Then she will have the time necessary to retrieve whatever firearm, if any, she decided on, as well as get the kids together, call 911, etc.
German Shepherds make a great family and protection asset