Be sure kittens are indeed abandoned before you disturb a nest… just because the momcat isn’t there now, doesn’t mean she’s not around.
If the kittens are clean, plump, and sleeping quietly in a heap, odds are they’ve got an attentive mom and should be left alone. Abandoned kittens will be dirty and the nest will be soiled, and they will cry continuously because they are hungry. Visit our guide to caring for orphaned kittens:
Ideally, kittens should not be taken away from the mother until 5-6 weeks of age, but if they are born to a feral mother, they should be taken away, if possible, closer to 4 weeks. At this age, its easier to tame them, and they should have already received antibodies from the mothers milk by that time. As they get older, it becomes increasingly harder to socialize and tame them, and kittens over 8 weeks old who haven't had any human contact will probably takes months to tame, if it can be done at all.
The kittens will need a foster home to socialize and care for them until they are social enough to be adopted and found a good home. If you are unable to socialize and care for the kittens youself, you can call your veterinarian who will have phone numbers to shelters and rescues in the area, who will hopefully have contact information for someone who can temporarily foster them. It is ideal to foster them yourself, because keep in mind that most shelters and foster homes fill up in the spring due to the high volume of kittens coming in. Vet clinics are usually unable to foster feral kittens due to the lack of time home environment and quality time that these kittens need to develop good social skills. If unsocialized/feral kittens are taken to a shelter, they will most likely be euthanized. However, good homes can usually be found for well-sociaized kittens.