Oh, that is so sad
I often see people with their dogs in their open yards with them and have always thought it would be nice to have a dog that was trained to stay in the yard with me without a leash. But I don't think I could EVER do that with a Yorkie or other small dog, because as you've witnessed today, you just never know what might happen....in an instant.
I am so sorry this happened to one of your babies and will say a prayer for her and for you tonight.