I'm sorry to hear you are still going through this with the babies. The sad truth is that most of the shelter cats and kittens end up getting sick. They usually don't have a very good start in life and its so much easier for them to pick up every little germ and virus.
I know this is very frustrating for you but just remember that you are helping these poor little ones get well and you will be rewarded with lots and lots of love for years to come.
Hang in there and let us know how the visit goes.