Oh, I'm so sorry! You sound as if you care so much about them, and I'm sure it's nothing you did wrong.

One question: did the vet run a blood test on the kittens? From my experience, vet checks don't necessarily include a blood test unless you specifically request it. Sometimes it's something that cannot be diagnosed without it. I would definitely have a blood test run on Bambino, if it hasn't already been done.

If you know who the momma-kitty is, it might be helpful to have her checked as well (if you can), and run blood tests on the others from the same litter, if possible. There are numerous possibilities, and sometimes the momma kitty can be a carrier of some disease and pass it on to her kittens. In this case, momma kitty should be spayed, to prevent another litter of sick kitties from being born.

Good luck with little Bambino. We have our paws crossed for him!