If there are fleas on the kittens, there are many more fleas in their environment. You could treat the environment with DE (diatomaceous earth). You can probably find DE at a local feed store, or sometimes at a health food store like Whole Foods. Make sure it's food-grade DE (it should say this on the label). There is another grade used for pool filters, and this will not work and is also harmful. If you can't find food-grade DE locally, you can order it online.
Food-grade DE is totally nontoxic--people even give it to their livestock and pets in their food (for intestinal parasites, and as a mineral supplement). It's a dry powder that dries out any kind of bug or insect if they have enough contact with it.
It won't work instantly, but it will gradually kill off fleas, ticks or any other bug where you have sprinkled it. You can also sprinkle a little DE in the kittens' fur. It will make the fur a little dry and tangly, temporarily. Don't get DE in their faces; it can irritate the eyes, and breathing in the dust is a bit irritating, too. So apply it gently so you don't raise dust. You could rub a little into their bellies and along the back. The same goes for you; it can dry your skin a little, or irritate you eyes/lungs slightly, so when sprinkling it around, do it gently so as not to raise dust.
If you apply DE where they hang out under the porch, spread it as much of the area as you can. It doesn't have to be in a thick layer--just a little is fine, like a sprinkling of snow. There are about 100 times more fleas off the animals than on them, so treating the area helps a lot.
You can also flea-comb the kittens. Feed stores and pet stores sell flea combs. It takes a lot of time, though--you need to go over the kittens pretty thoroughly. Some longhaired coats are hard to comb through, and if you put DE in their fur, the tangles might make it impossible to comb them. But if you can, take each kitten and comb, and check after each pass through the fur. If there is a flea on the comb, dump it into a pan of water with a little soap or detergent in it--the flea will drown right away. Keep the bowl or pan next to the kitten. Keep combing till you don't get fleas. This needs to be done on each kitten, every day.
Another way to treat the kittens is to bathe them. Any mild, plain soap will kill fleas on the kittens, if you leave the suds on for five or ten minutes. Not easy with a squirming kitten! But you can use very mild soap--anything gentle made for babies. You don't need to use a lot of it, either. When you wet down the kitten, wet the head and neck first. Soap up the neck and the back of the head, avoiding the face. That way, when fleas run for higher ground, they won't be able to avoid drowning by getting up on the cat's face. Then wet and soap the rest of the kitten, and you can keep rubbing in the suds for five minutes or so, or just hold the kitten. Make sure you soap the belly area. The fleas on the kitten should be drowned in five minutes or so.
I like to use Dr. Bronner's liquid soap for babies--it's unscented and gentle. Another plain castile soap would be good, too. Try to avoid perfumes and definitely don't use anything with strong essential oils in it--those oils can be very toxic to cats (like tea tree oil, eucalpytus, lavender, etc.). There are pet shampoos with these oils in them, and some of them are labeled as flea shampoos. Essential oils can kill cats so use something plain very plain. You could a simple bar soap that has no perfume.
You don't have to use anything strong like Dawn, or something with citrus in it. Some of the citrus shampoos have d-limonene in them, which is made from citrus peel. It will kill fleas, but can be very toxic to cats. Stick with something gentle and unperfumed.
There may be rescue groups in your area that can help you with the kittens, and neuter the feral cat. They may be able to do that, give her shots, and then release her again to your yard, or find another home for her if that's what you want. You can check at Petfinder.com--they have lists of local rescue groups you can contact.
People have suggested calling Animal Control or your local shelter, but you are wise not to in some areas. The suggestions are right about some shelters, but not about all of them. Many shelters will euthanize almost all animals, and some don't give any health care or neuter, and don't find homes. It all depends on their capacity. Check Petfinder.com for local rescue groups first. They may be able to help you, and rescue groups usually don't euthanize.
Thank you for taking care of these babies!
J.