Kittens are supposed to stay with their mother until they are at least 10-12 weeks old for many reasons. Before they are fully weaned, which occurs by about the 8th week of life, they drink milk from their mother. By doing this they receive passive immunity from the mother. This protects them from disease and illness temporarily until their immune systems are strong enough to protect them. The mother also nurtures her babies, giving them the love, attention and warmth that little kittens need. They also learn a lot from their mothers. Around the 6th week of life, the kittens and mom will start to play together. This will teach kittens how to hunt, fight, how to be a well-behaved social animal, how to jump, balance, and so on.
And never, never give kittens away for free. By that I mean never advertise them as "free to a good home". There are too many sick people around that just wait for an opportunity like that. I have heard too many horror stories of what terrible thing can happen to the little ones. If someone is really interested they are more than willing to spend a nominal amount on the adoption. Also, be sure to thoroughly scrutinize each and every person looking to adopt your babies.
Hope this helps and good luck