agreed with 10-12 weeks, they should even still be allowed to nurse if they like to until that time, even though tthey are weaned and eating solids. Mom teaches them all sorts of cat things, if taken too early, they can have behavioral problems later on in life. The mother can however be spayed while the kittens are still with her at about 6-8 weeks or so. You can even have the kittens altered too before going to new homes so then you will be sure they won't be allowed to reproduce.
Also I would NEVER give the cats away for free unless you know the person well. They can be collected to be sold to labs or as bait in dog fighting, or even just to irresponaible people.