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 I’ve 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 doesn’t 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 I’d rather give them to people who will provide a good home. I live in Dallas. Please inquire if you are interested!