Thank you everyone for the thoughtful responses.
My own HD weapons are a 12 gauge pump (Mossberg 500) and a .357 revolver (S&W 66), but I'm bigger, am much more familiar with firearms in general, and don't have kids, so I didn't want to assume that my choices would be good for her.
I should mention at this point that the first question is "if" she will get a gun. "Which" gun would be a secondary consideration. I don't know if she would have the right mindset to use a deadly weapon or the dedication to become properly trained with one. And maybe the challenges that arise from keeping a gun in a house with kids around would outweigh the benefits for her.
I have some rifles, shotguns, and handguns, so I might just take her shooting and she can decide if it's something she'd like to pursue further. If so, there are defensive firearm classes offered at the local Cabela's (some specifically for women and others for anyone).