Most kittens are ready to leave between 6 and 8 weeks.
^ weeks is a bit early I think , although I had a kitten at 3 weeks old as it had been rejected by it's mother .
You will find that the mother cat will wean the kittens herself, and if there is a VERY shallow litter tray next to the box they live in she will even toilet train them.
I was amazed to see my cat do this to her 4 kittens at a very early age.
They were sleeping in a crisp box - the sort that holds 48 packets - with the side cut down.
The litter tray provided was a swiss roll baking tin (which obviously had to be thrown out afterwards)
One day when a kitten weed the mother cat clipped the kitten on the head and the poor thing bounced across the box, then the mother picked the kitten up and put her in the litter tray.
I have never seen any animal toilet trained so quickly!!
Enjoy your babies !