I used to work for the JCP portrait studios. The JCP and Target studios are the same company (they just rent the space in the stores).
Like PP said, you are going to be the last appointment of the day (they do this for both safety and ease), you'll have to pay a sitting fee at the JCP (Target doesn't have sitting fees, so that will be the way to go... even if you purchase the Portrait club card from JCP, it won't cover the dogs, unfortunately), and you'll have to sign a pet waiver (which isn't a big deal, just something saying that if something happens to or because of your pet, they aren't responsible).
The Target store in Boca Park usually isn't that busy, so you might be able to get in there easier than any other location in the valley.