Hello, my kittens are 6 weeks old, I think it is still early days to separate them from the mum cat. But when time is due, how do I do it? will the mum get depressed, will the kittens get sad?

Opinions vary. My opinion is usually the mama will start to wean them herself somewhere between 6 and 10 weeks of age. This opinion is from finding kittens, associating with and working with groups that foster and talking to different vets and techs. I will mention 1 vet tech said 3 weeks but I though she was full of hooey.
The kittens should be offered moistened kitten food at 3 weeks of age.
Mama should be eating kitten chow as well, nursing takes a lot of nutrients from her.
Yes mama will miss them, look for them, and call for them. They will miss her as well, but all that I have found seem to do OK snuggling up with a human.
Last but not least please get her spayed or she will be having litter after litter our shelters and streets are overflowing with unwanted cats and kittens. Sadly, some of them will be killed because there are not enough homes for all of them.