There are no specific things to give nursing cats (referred to as queens) that will increase milk production. To determine how to increase the milk production of a nursing cat, it's necessary to identify any problems the nursing cat is having. The best option is to ensure the queen is healthy and receiving the proper nutrition.
A queen requires double or triple the amount of calories that a normal, non-lactating cat would require. In order to produce an adequate supply of milk, or even any milk at all, a cat requires extra calories. Ensure the cat is receiving as much food as she seems to require and that she is well hydrated. Another way to increase milk production is by having the kittens suckle the mother, which stimulates her milk production.
If the cat is receiving an abundance of high quality, nutritious food and is still not producing milk, there may be a more serious problem. Mastitis, for example, occurs when the mammary gland is inflamed, blocked or infected. If a queen appears to be in pain, if the milk is discolored, or if her glands are swollen, she may be suffering from mastitis. Additionally, if a cat has an unusually large litter, she may not be able to keep up with all the milk required. In these cases, a veterinarian may prescribe a medication designed to increase milk production or a medication to supplement the litter.