There are several threads already more or less completed and chuck full of advice on this site -- one on Newborns in particular, but while dealt with bringing them to maturity. One on a 7-day old...
Your vet should have recommended the appropriate Science Diet (or other comparable type) canned food for sick cat, newborns, and kittens that need to be weaned. Maybe in the rush of things, he forgot. what does he say (or she, as the case may be) your cat suffers from? Viral infection? Parasites? Milk Fever? Everything depends on what is affecting her, since all infections and parasites will manifest in the kittens.
By all means, if she is not infectious, take on the kittens' total feeding, but let them nurse their mother for one or two short periods during the day (or perhaps leave them with her at night for comfort and her care) and keep them apart during the day. You can help to prematurely wean them by combining canned kitten food with your catmilk formula -- start with a half a teaspoon in a maybe three tablespoons of canned food, mash up and mix well, and put this in a jam-jar lid. Introduce the kittens one at a time to this by gently shoving their faces into it. They will clean it off their faces (if the cats are self-grooming well, you can try putting their forepaws in the mixture and setting them aside to clean their paws) -- they will get a taste of the food. Usually you only have to encourage them in this way a few times before the start biting it up -- they generally don't know about lapping yet, although there are always the naturals. You should be able to feel all of them around a single dessert plate very quickly. Keep on offering the bottle (or perhaps you are using a syringe) until they clearly would rather have the goo in the jam jar lid. Every day, increase the amount of canned food to the amount of milk formula. Ask you vet when you can use cow's milk or human baby formula -- they are cheaper. But remember that young kittens can't tolerate cow's milk well, and it takes less time than you can drive to the vet's for a little kitten to simply stop living.
At some point, the kittens will attack the food and leave the wettest milky part in the dish. They are developing teeth now and you can offer them boiled chicken (if you boil it in prepared soup mix, make sure it is one that is very, very low in sodium), only remember to cut it up into very small pieces (the quality of hamburger). Keep up the canned baby or kitten food at the same time. Most animals will signal you quite determinedly when they want to change to more grown-up foods.
Kittens should be kept on kitten mixtures (whether canned or dry) until they are around 4-8 months old.
As to your poor mother cat, when you find out what she has and why she is ill, there are all sorts of advisors on this particular category.