Hi...One of my queens had mastitis last summer. Her four babies had just turned 3 weeks old and I noticed she was not lying down and feeding as she should. The minute they would start nursing, she would get up and walk off, leaving 4 howling babies. I checked her out and discovered the problem...mastitis. I mixed up some carnation milk (half and half with water) plus 1 tbsp or so of corn syrup, warmed it and poured it in a low sided bowl or shallow plate, and then mixed in some rice baby cereal (the first start one) and offered that to them. They ate a box of this cereal in no time. Poor mom, had hard lumps around her teats and was put on antibotics right away. She never did nurse them much after that. She is pregnant again now and I sure hope history does not repeat itself. I find that pablum (rice cereal) is excellant for tiny kittens. If they refuse to eat, you will have to bottle feed. Good luck, let us know how you make out. The mom probably will not nurse as she is sore. It probably is better for her and babies if you supplement them with the cereal milk mixture. I am feeding that now to my Chloe's 4 week old babies. They simply love it!
I just found this info on the internet.....

"Mastitis is a medical emergency, and you should take your cat to the veterinarian immediately.

There seems to be two different schools of thought in regards to allowing kittens to nurse from a queen with mastitis. Your veterinarian is the best person to advise on how to proceed, if you should permit the kittens to continue to nurse from the affected gland(s), or from the queen altogether. It seems that the decision is based on several factors: if the mastitis is confined to one gland it may be recommended that the kittens continue to nurse from all but that gland, the physical condition of the cat, and if the mastitis is septic or non-septic. What does appear to be commonly recommended is to avoid kittens nursing from a gland which has become infected, as infection can be passed onto the kittens, which can result in sickness or death. Only your own veterinarian can recommend which is the best & safest way to proceed.

If your veterinarian does recommend kittens cease nursing from the mother it may be temporary until she can recover, or permanent. Either way, if the kittens are too young to wean, you will have to bottle feed them with a specially formulated milk designed for kittens. Cow's milk is not an appropriate substitute for kittens."

I disagree about the cows milk not being good for kittens....straight cows milk may not be ok but the carnation milk sure is. It is canned and it is very much ok. I just raised two orphan barn kitties from the age of 7 days...and I did it entirely on a carnation milk formula. They are 7 weeks old tomorrow and are doing great.