I believe that a .22, and especially the 15-22, would be a great HD firearm. Adequate capacity, fairly maneuverable in close quarters, and rails for accessories make this a good choice. Also, don't forget that it will be easier to educate and train the wife since the recoil and noise is only a fraction of what it would be with major caliber weapons. My EDC is a Ruger LCP and I sometimes get a little grief from my friends for it "only" being a .380, but so far, no one has offered to take a round to prove how ineffective it would be in an emergency. It is infinitely better to have some type of weapon handy in case of an emergency than to have to rely on voice commands to scare away bad guys. With a new baby on the way, your first concern should be to secure that gun against unauthorized access now so you will be in the habit by the time they reach the crawling stage. It won't be as long as you think.
RichH