Oh I'm so sorry. For you and for Kia. I too am a single mom of a dog. My boyfriend and I were together when I got Duncan so they have grown very attached. I worry about how Duncan will feel if/when something happens and we are no longer together. But I also know that Duncan (and Kia) love their mommys the most! She'll be sad because she'll see your sadness and anger and hurt. Your responsibility to her is to make this transition as easy as possible. She is living in a new home, her routines will change, her "family" has changed. Spend lots of quality time with her and try to keep things as close to normal as you can.
This is so strange that you wrote this today because my boyfriend and I are in the middle of "discussions" over our future and I couldn't sleep last night worrying about Duncan! Not only is he attached to Collin, but since I work 2 jobs, Collin has really been a big help to me taking care of Duncan. But, we're strong , independent women and we can do it!!
I am so sure that others have much better advice for how to help Kia deal with this but I wanted to let you know that I understand where you're coming from completely. Good luck! Hugs to you and Kia.
If you need to talk, you know where I am!