No need for Vet ( not now ), it's natural selection, or...
did you touched the kittens? ( cause my cat kills the kittens in the first weeks if someone played with them, it's the smell )
Like someone said if it's the first time 90% will die.
Maybe they are dying from dehidratation ( i really dont know how to spell that - lack of water in the bodie), put some soro ( another word that i never used before ) that water thing that in the hospitals thay tube you with, in a piece of coton and see if the kittens absorb it ( yes , in top of the fur ).
Or ( and this is really bad ), the mother ( or the father ) have cat's aids, in that case call a vet ( but IMO you should try my above statements).
I'm cat guy, never had a time in my short life ( only 28 ) without cat's ( and dog's, and fish, a bird, i even had a spider once but she escaped the terranium and pissed my wife)