There is a big debate on when kittens should leave mum, and generally I believe it works on a cat by cat basis. I have previously worked in a rescue shelter, and as long as kittens are toileting normally by themselves, and are fully weaned onto solid food, then we would often start splitting them from mum over a two week weaning period at around six weeks of age.
As long as the kittens are putting on adequate weight and doing the above mentioned toileting and eating activites healthily, then there will be no real health detriment. Moving into a new home at six to eight weeks of age can have a very positive effect on a kittens social development. During this period of their life they are very open to experiencing and accepting new things as the norm, so a new home will be more easily accepted by an 8 week old kitten than one rehomed at 12, as this one may have already gone through a lot of it's social development and find all the new sensations overwhelming and quite difficult to cope with.
From the sounds of it, your kittens will be doing fine. The big deciding factor we used to use is whether or not the kittens were vaccinated, usually the first dose should be given no earlier than seven weeks old, so any time after that if the kitten is healthy should be fine.