Laura, my heart breaks for you!
I wish that I could say that dogs are stupid and don't have feelings, but that would be lying to you. They do have feelings and unfortunately, sadness is one of them. Sure, this little girl is going to miss you terribly. You've been the one to take care of her and have given her the love she's needed all this time. While the seperation will be somewhat easier since she knows your son, she still sees you as mom.
Maybe you can have visitation rights. Or, you could take them to court and gain custody of this little girl.
I have a feeling you wouldn't do that, however.
We're all here for ya. Take it one day at a time.