Originally Posted by Slims

Um, do you want a gun for home defense, or for concealed carry?

If you want a gun for home defense, you are probably best served by getting a shotgun.

But since you asked for handgun advice, here goes:

For the home, bigger is better as far as gunsize is concerned. I would reccommend either a fullsize glock in either 9mm or .40 s&w or a fullsize revolver (6 inch barrel) in .357. Anything bigger bulletwise and you and your gf will probably have a hard time learning to shoot it. You should get a fullsize pistol because they are easier to shoot, more accurate, have more oomph, and are easier to control than the smaller weapons which are made to be concealable. Unless you and your gf are willing to spend endless hours at the range mastering a particular weapon, simpler is better. There are no safeties to worry about on glocks or revolvers, so you just point and pull the trigger, which means you are less likely to screw something up in a panic situation.

As for concealed carry, well, there is a lot of subjective reasoning and personal preference that comes into play.
Essentially, you have to find a balance between concealablity and performance. Bigger pistols are harder to conceal, and smaller ones are harder to control, and typically don't perform as well.
I personally carry (and love) a HK USP Compact in .40 S&W, but it is fairly bulky and hard to conceal. I am thinking of purchasing another, smaller pistol for the summer months.
I think the general consensus among the shooting community is that you shouldn't bother with anything smaller than .380 (except for very specialized purposes) or with anything larger than .45 for concealed carry. You should probably look into something in 9mm or .40 s&w. Kahr arms is an excellent brand, and makes a bunch of very excellent, concealable weapons.

You will need a gunsafe (to keep it from your kid). They make specialized pistol safes now that are specifically designed to be opened almost immediately when you need them too. Most of them are fairly cheap, and it is worth the expense to make sure an accident doesn't happen. Just hiding the weapons isn't good enough, trust me.