Kittens can quickly succumb to flea anemia so this very well may be your cause if the kittens have fleas. I suggest that you see the vet immediately for some flea prevention for the remaining cats. Capstar can be used on young kittens and will eliminate the fleas within 3 hours of its use.
Aside from that, you also have to worry about parasites, FIV/FelV and other contagious diseases that come with cats that have not been vaccinated.
No matter what, you need to get some capstar from the vet and start using it. You will also need to treat the sleeping areas and then go from there.
Let me know if you still have questions.
