Essential fatty acids (like omega 3 and omega 6 fatty acids) and fat soluble vitamins (Vitamin A, Vitamin E, Vitamin, D, Vitamin K). The plain fat is a nutrient too.
Kittens, generally, can begin to use a litter box at about four weeks of age, and a kitten being raised without its mother or adult cat will take slightly longer to litter train; so, keep this in mind when you begin the task of litter training your kitten. - Place the kitten in the box 1/2 hour after play or meals.
Somewhere around 5 or 6 weeks of age, you'll have a nesting box full of kittens that are learning to run. They'll be playing with their littermates, pouncing on momma kitty and giving you plenty of entertainment. Since they're fully mobile by this time, they can get into all kinds of mischief.