Remember that the BEST option is to always leave the kittens with their mother.
The best thing for the family is to leave the kittens where they are for now as long as the location is safe. Remember, the mother is best able to care for her kittens. You can assist in providing food, water, and shelter. Monitor the family daily and make the environment as safe for them as you can.
When the kittens are out exploring on their own, this is the optimal age to bring them out of the field and to a shelter for socialization, health checkup and spay-neuter.
After you bring the kittens in, many shelters like Charleston Animal Society, will provide a trap for you to catch mom. She can be brought in for spay-neuter and then returned to the field.