This is just so sad! But I know what you are going through! My mom and I(years ago) had the same problem, with a cat we found. After she gave birth, she also did not produce any milk, and the kittens started dying. We took them to the vets like you did, but it was too late! You are doing the best you can, it is very heartbreaking, but just concentrate on the little one left! It is all you can do, there was a reason why the others didnt make it, and it is out of your hands. You gave them the best that you could, and I am sure they all thank you! Especially, Sweetpea! You are her guardian angel!