You should observe the mother cat's routine for a few days including to see if she is stimulating them by licking their bottoms. If she is not feeding them you need to get to a pet food store or even walmart to buy a can of kitten formula powder or liquid and a couple of kitten bottles sold at the same places. Read the directions carefully and feed each kitten every 2-3 hours you will be able to tell their needs after a few days. I know all of this because I went through it myself with a litter of 4 when they were only 4 weeks one of their mother's nipples became severely infected causing her milk to be infected. So, I was mom for the next 3 weeks while their mom recuperated. If all else fails contact a local veterinarian. Good Luck.
You will know when the kittens arent growing and still arent like newborns, and are flopsy and slow.