Wet food twice daily for adults, for kittens 3 times a day as much as they want to eat. Look for foods where the first ingredients are MEAT, not meat-by-products, and not carbs like corn, rice etc. carbs should be later on the list.
The only barely decent food you can buy in a grocery store is Iams. The rest is junk. Good foods in pet stores include:
Spot's Stew among others.
But, no matter how good a food is, it doesnt do the cat or kitten any good if they don't eat it. So the first thing is, make sure they are eating their food. If they wont eat the good stuff, then get something else, and then slowly start adding in the better food.
Dry food should not be the primary source of calories.