You can get a product for feeding human babies called farax it is high in protein and iron I would suggest to use that I am actually feeding week old baby birds that upon advice from an avian nurse mix it with cat milk (you can get that from any grocery store, if you can't get it milk from your fridge will be ok but just remember cats are lactose intolerant so do try to get the cat milk first) make sure its runny and see if she will drink it by herself (place her in front of the dish) as time goes on you can thicken it to make it heavier also some very soggy weet-bix are good too try not to feed her cat food until six weeks of age but if she wont eat anything else try it with any advice I give I suggest to talk to your vet first by phone
good luck
good luck