From the time they are born until around 3-4 weeks later, kittens only drink their mother’s milk or formula milk. After that, they should learn to eat other kinds of food.
Mothers would naturally help their kittens get used to more solid foods when they try to imitate her eating her solid cat food. You could help the process by providing pate-style cat food, which is safer and easier for the kittens.
If the mother isn’t around, you can help kittens transition to solid foods by serving them a shallow bowl with a bit of pate-style canned cat food mixed into their milk formula. You have to spoon-feed them their meal and bring them closer to the bowl as they get used to the new food and the new way of eating. Once this messy step is over, the kittens should comfortably eat independently and the bottle-feeding stage has passed.
Your pet clinic is a valuable resource when it comes to issues that affect your pet's health and well-being.