Is your momma cat thin by chance?
It takes a lot of calories to feel well enough and strong enough to nurse kittens. You should be feeding your momma cat a high calorie good food, and you can get something like that at a store like Petsmart, or Petco. I would give her both canned and dry food, and maybe if she is eating well and taking in more calories, she may nurse better. The KMR that ZooCVT mentioned is a good alternative, but if you can get the mother cat to feed the kittens, it will be better for them.
The following is taken from the website by Mike Richards, DVM:
Mom cat losing weight
Q: Dr. Mike, Our one year old calico cat had 5 kittens six weeks ago. Three are still with her, not having been given away, yet. So, she is still nursing them on occasion. We have noticed that even though she seems to have a good appetite and is always out of her room crying for food, that she has gotten very thin. We first attributed her weight loss to her young age, and the strain of 5 nursing kittens, but now are wondering if she might have another problem, maybe worms or something more serious. No worms have been noted in stools. It also seems that the area around her nipples seems warm to touch, recently. Maybe just my imagination, I don't know. Do you have any ideas?Thanks for your time
A: It takes an amazing amount of energy to produce the milk for five kittens. If you have not been feeding your cat a high calorie food (like a kitten food) and have not been feeding her as much as she wants, it is very likely that she is just losing weight because she is not getting enough calories to make up for the milk production.
Many cats who have kittens do have a reactivation of roundworm migration and subsequent roundworm infestation in their intestinal tract. It is usually a good idea to deworm a cat after it has kittens with a dewormer that kills roundworms. The safest of the over-the-counter dewormers are the ones containing pyrantel pomoate as the active ingredient.
The warmth around the nipples may just be inflammation associated with the decrease in nursing and development of teeth in the kittens. It can be the precursor of mastitis, though. Watch carefully for hard swellings, bruised looking areas around the nipples, excessive heat or pain. If any of these symptoms occur, have your vet check your cat.
Mike Richards, DVM
God bless and good luck with your kitties! p.s. Keeping them inside your home only, as well as the mother, is the best thing also, for them.