Found this on the internet - hope it helps.
Curing Fading Kitten Syndrome
In my 35 years of breeding Siamese cats, I often have experienced what many people call "Fading Kitten Syndrome".
The characteristics are as follows: At about 7-9 weeks of age, a litter of kittens, which has been eating vigorously from a dish, suddenly does not seem interested in food any more. It may just be one or two kittens in the litter, or the whole litter. No upper respiratory symptoms are evident. Three or four days of not eating causes dehydration and can lead to death.
I discovered many years ago that in such cases, if antibiotics are given, eating resumes 90% of the time within 24-36 hours. Originally I used tetracycline, but in recent years have used doxycycline as it does not seem to yellow the teeth as does tetracycline. I make up the antibiotic by dumping a 250mg capsule into a mortor and add about 3g of a cat vitamin powder for flavor and grind with 10g water to get a slurry. I give each kitten 1/4 ml. twice a day by eyedropper. They tolerate it quite well.
Some people have told me they get the same results with amoxycillin.
Obviously, if the kittens had URI symptoms you would give antibiotics, but in a situation like this where the only symptom is loss of appetite, people just don't think to give antibiotics.
Disclaimer: I am not a vet and have had no medical training but have spent 30 years as a research scientist. This is the procedure I have found that works for me.