The problem is two-fold. Kittens should be at least 12 weeks old before they're separated from their mother, so they'll need to stay together until then.
On the other hand, the longer you wait to handle feral kittens, the harder it'll be to tame them.
I'd suggest capturing them all at once--the mother and all eight kittens--and taking them all to the SPCA. That way they can stay together and the kittens will be safe while they grow. Contact your local SPCA beforehand to ask about their euthanasia policy. Many clinics are overburdened with animals, and while the kittens may be adopted out with ease, finding a home for a feral adult cat will be much more difficult. For example, the SPCA near my hometown has a 2-week euthanasia policy, which in my opinion is much too short. If your SPCA's pet-keeping limit is too short for your tastes, ask if it would be possible for them to contact you after the kittens have been weaned and the mother cat's chances for adoption are running out. That way you can ensure that the kittens are properly cared for and adopted, and that the mother cat has a sure-fire home if no one else wants her.