Depending on the breed and how many you've got, I'd let mum still feed them if she wants to until they are about five weeks by which time they will be virtually fully weaned anyway. As long as she can get in and out she will let them feed until she has had enough of them, she will soon let you and them know when they are getting too much for her.
You can introduce any sort of puppy food. I always like them to try all sorts, dry, tinned, raw tripe, so that they willl be used to anything their new owners want to feed. Be careful with cows milk, it is very rich but if you can get goat's milk that is the nearest to their mum's milk.
If you are going to give them a complete food, start off soaking it, then gradually feed it less and less soaked until it is completely dry, but make sure they always have access to water.