Put in a little of the cat food in with the kitten food your kittens are used to and mix all together. You can also offer both foods separately so that they can try the new cat food. This done over 7 to 10 days while gradually increasing amount of new food, will soon lead to a complete change.
When we wean kittens, we make a soupy mix with kitten milk replacer and canned food. Then over the next few weeks add in crunchies until they get used to it. Kittens need kitten food until they're about a year old.
Mix soft cat food and dry kitten food or use kitten formula in the hard kitten food to soften it. Mom can eat kitten kibble too. Don't switch then to adult cat food until they are adult cats. they need the extra vitamins and fats in the kitten foods.