What is the best food for a cat to gain weigh after nursing kittens?

A mom cat and her kittens adopted my patio a little while ago, and naturally Ive been trying to leave some food and water out for them as often as possible. The mom cat was very skinny so I have been feeding her extra foods packed with vitamins and fats, like chicken, special dry canned cat food, turkey, some beef, and she LOVES biscuits and cheese! :) But she never seems to gain any weight. I want to make sure she is eating enough to produce milk for her kittens and remain healthy, but it doesnt seem to matter what food I give her. Any food recommendations for a nursing stray cat? Also... If they stay around any longer, I will have to take them to the shelter but Id rather give them to people who will provide a good home. I live in Dallas. Please inquire if you are interested!