This is hard for me to answer. My first choice would be my 870 or Mossberg Preacher gun. But since over penetration is an issue, my first shell would be birdshot followed by buckshot, probably 00. This might be lowered. Next consideration is accessibilty, for me this would be younguns getting their paws on it when I'm not there. I could put a life jacket on the shotgun and put it under the bed. Then I'd have to fumble with keys. So handgun rules under these circumstances. I have one of those door ejecting type of safes where you just push buttons, no key needed.
Now the problem becomes which handgun. I have two semi autos, an H&K .45 USP Compact and a Beretta 9 mm 92FS. I'm ruling these out because they can possibly jam. Whereas that ain't gonna happen with the revolver. Not my first choice but it's what I own, a 45 LC Peacemaker. Need to get something more 20th Century but that might now happen soon. (Promised myself that my next purchase is a Wilson Combat .45, so I'm in a saving mode.) Over-penetration is an issue, but I'm using this as a cover weapon to retrieve one of my shotguns that are nearby.
I'm ruling out the AR15 because, first you better be on target, and 2nd over-penetration. At 25 yards, I can put a smiley face on you with the Peacemaker. I'm just as good with the other two but I was made aware of a couple of drawbacks with semi-autos. The clips can retain a memory if you leave them loaded for an extended period of time. This can cause jamming. Next was a story about a guy being robbed by two guys with semi-autos. Both of their guns jammed in the attempt and he used his revolver and killed them both.
I've reinforced the walls in the kids room with bookshelves (loaded with books) to reduce the overpenetration issue. I also plan on shooting high (head shot) to take away additional danger. My worse case scenario includes moving to the stairwell to cover anyone coming up the stairs and then having the kids retreat to one bedroom along with my spouse. My dog and I will battle it out with the oncoming foe.