One, the 12 gauge should be the first line of defense. find a short barrel for it, or find a used 30 inch barrel fixed choke around town, and cut it back to 19 inches from the closed bolt face to the end of the barrel. (I know the law is 18, but 19 gives some leeway in cleaning up the cut. and the one inch won't matter)
Two, get them shooting. get them used to the gun.
three, if there is no one else in the home, the gun goes in the corner nearest your dad's side of the bed, mom gets the 9 and the phone. No reason to hide a gun if you have no little kids around, if they are worried about appearances, hang a shirt over it.
how big is the house? how close are the neighbors? I would find a box of T or F shot. 2 3/4 inch steel. or 4 buck or BB in lead. anyone who says that is not enough inside a house has not seen first hand the effects of it. PERIOD.