If you find a litter of kittens and they are 6 to 8 weeks old, they are old enough to separate from their mother and surrender to a shelter. If you need help determining a rough age for kittens check out this image and link from Alley Cat Alleys.
Generally the cats will be wandering around a bit during the day while their mom hunts. You may be able to just pick up the kittens if they're friendly enough, but often with outside cats you'll need to trap them. If you have multiple traps you can often trap an entire litter in one evening if you pick up the trapped kittens and move them somewhere safe. You'll often have the best luck trapping closest to the den they stay in during the day. Move the filled traps away, if you have multiple, and replace it with an empty one.
Often, once you have success trapping a mom will move them away for a little while and bring them back in a few days or even weeks. It can take persistence to catching an entire litter.
Upon capture, know that the cats are a transmission vector for both disease and parasites. Definitely keep them separate from your other animals during this initial period. If you can, you would do well to give each a kitten size dose of a wormer and flea/tick medication. If you can't get that, each kitten should at least get a bath in warm water and dish soap, and then a brush with a flea comb. Ideally, do it in a bathroom with a closed door. They will likely want to escape given the chance. Keep them in traps, cat carriers, a cage, or a large cardboard box and make sure to provide them food and water.
Once you capture them all you or someone you know needs to foster them. The kittens may not trust humans immediately, so set them up in an isolated room in your house. If you're interested in doing this often, for a shelter, you can get a multi-level cage to keep them in when you aren't actively socializing with them, cleaning, or feeding them.
Kittens, when fostered, quickly become fun to be around but the transition isn't instant. If you keep at it, the kittens will grow to enjoy humans. Play with them as much as possible, feed them regularly, and keep them clean and healthy. If there seems to be an issue, get them to a vet. Begin working with a shelter to surrender them or foster them for adoption.
Central Iowa resources for kitten surrender and fostering:
Animal Rescue League - https://www.arl-iowa.org/pet-help/pet-helpline/
Cat surrender, fill out one page per kitten. ARL will assess the application and call back. It helps if you can somewhat tame the cats and ensure they like to be petted, picked up, held, and played with. The ARL will not take a cat until they are 8 weeks old. When surrendered the ARL will post them so fosters can volunteer to take them in and further socialize them.
Furry Friends Refuge - https://furryfriendsrefuge.org/about/contact-us/
Furry Friends prefers cats to come to them at 7-8 weeks old if previously feral. They need to be tamed enough to be held and be petted as they’ll most likely wind up in a foster home.
Rescue, Rehab, Rehome - https://rescuerehabrehome.org/feline-contact-us
RRR also prefers younger cats, if they come from a previously feral life. They need to be tamed enough to be held and petted as they’ll be placed directly into a foster’s home.
Aheinz 57 - https://www.aheinz57.com/owner-surrender-resources/
Hope Animal Rescue of Iowa - http://hopeanimalrescueofiowa.org/pages/624/surren
Hope, like many shelters already listed, doesn’t have a physical shelter. All animals are fostered and ideally should be tamed already so they can be held and petted.
Happy Tails Ranch and Rescue - https://happytalesranchandrescue.wordpress.com/contact-us/
Animal Lifeline of Iowa - https://www.animal-lifeline.com/contact
Animal Lifeline specializes in special needs animals which includes: "injured, handicapped, have a serious medical condition, are abused, are pregnant, are nursing moms, or are orphans that require bottle feeding"
For More Information
On trapping kittens, click on the orange kitten
For more information on fostering kittens, click on the tortoiseshell kitten. Bear in mind the guide is specific to a shelter in Columbus, Georgia. If you are working with a shelter, make sure to contact them! I personally think some items in it are a little overkill like dumping litter every two days. Also, I wouldn't advise trying to foster a feral mom and her kittens together unless you have an established, friendly relationship.