Hi Mary -
Kittens are a wonderful thing. Aren't they adorable?
It's best to keep the kittens with the mother until they are at least 8 weeks old. Many people separate them earlier than that, and while the kittens will survive and grow up to be fully developed cats it really isn't until after 8 weeks that they are ready to be separated, both emotionally as well as from a developmental level.
Often times cats that get separated earlier than 8 weeks are a bit more insecure, and will tend to be shyer at first.
If you can, wait that long. Regardless of how old they are though, there's nothing to prevent from the separation. Even at 8 weeks when you remove the kitten it will most likely meow and wonder for a few days looking for its brothers and sisters as well as for its mother.
That's totally normal, and within a few days (even hours) the bond between the owners and the new kitten begins to form and eventually the cat replaces that relationship with its owners and 'forgets' its mother and other kittens.
Best of luck, Itay.