[Astronaut]

H&K USP is a very fine gun. H&K has a new model called the P2000 that you might want to look at, if you can find one.

Glock and Sig Sauer are also excellent choices. Ultimately, it has to be something you are comfortable shooting.

[Willie Green]

Ultimately, it has to be something you are comfortable shooting.

I certainly agree.
And while a .45 certainly has the stopping power for self-defense, it is my personal preference to use a caliber with a little less oomph, simply because it's easier to control.

Have you considered a 9mm or .380?
Something along those lines may be better if you wish to further convert "she-who-must-be-obeyed" and teach her to use it for defense as well. Just one more step in winning her confidence and acceptence before embarking on training the kids.

Just a suggestion.