Do you have a mama with your kittens or are they bottle babies you are raising? When I have a mama, I wait until the kittens are showing interest in her food and then make them a separate dish, usually at 5-6 weeks of age. I have 2 kittens and a mama now and one kitten is just now interested in mama's wet food, which she gets twice a day, and the other is not. Mama does not appear to like the kitten getting into her food either!
With bottle babies, again I don't start until about five weeks because once you start solid food, they start pooping solids so I want them to be old enough to learn to use a litter box. Mine have been using a litter box with corn cob litter (horse bedding actually) to pee in for a couple of weeks now. Mama was not litter trained when dumped here, so I have had to put six litter boxes at one end of the room because she kept making her piles on any bare floor that was available, so now there is no bare floor and she is using the litter. The kittens caught on too.
With bottle babies, again I don't start until about five weeks because once you start solid food, they start pooping solids so I want them to be old enough to learn to use a litter box. Mine have been using a litter box with corn cob litter (horse bedding actually) to pee in for a couple of weeks now. Mama was not litter trained when dumped here, so I have had to put six litter boxes at one end of the room because she kept making her piles on any bare floor that was available, so now there is no bare floor and she is using the litter. The kittens caught on too.