you can start, they will begin to imitate the mother. Don't expect them to just right up and gobble it down or anything, you might even end up throwing a lot out, but introduce it and wait and see. The mother I hope is still getting wet and dry kitten food right? Just feed her near the kittens and they will eventually wonder over and snack.
Feed them a high quality kitten food with no by-products and meat as the first few ingredients and you should be fine. They will only snack on wet food now though, don't expect them to eat dry yet.
You do have a litterbox down for them too right? They need something with low sides, cake tins work great. Also a non-clumping litter. I know that wasn't your question but one thing just leads to another, you know how it is