It’s sweet of you to feed them, but truthfully, that will only encourage them to stick around, grow up, and have more kittens. Within a year, you’ll probably have 15 or so hanging around your yard.
As far as shelters go, unless they are adoptable, most no-kill shelters will not take them. Shelters are in the adoption business, not the sanctuary business. (Oh, there are a very few that provide sanctuary to unadoptable cats, but not many.) If they are feral, they are almost certainly not adoptable - possibly with many weeks or months of socialization they would be, but the adult probably never will be.
If you are able to trap them, a shelter might take the kitten if it’s young enough to be socialized to humans. It would be great to have the adult spayed and then release her (this is called “TNVR” - Trap, Neuter, Vaccinate, Release) - but I only advocate this if the animal is part of a monitored colony. The other option is to trap them and take them to a kill shelter, where they will most likely be put to sleep.
I know what you are feeling - I have dealt with hundreds of feral cats and kittens - some I’ve taken in, some I’ve socialized, some I’ve been successful in adopting out - and some I’ve had put to sleep. It’s heartbreaking. But sometimes that’s the only option.
If you do decide to trap them, you might be able to just pick up the kitten (wear gloves) but the mother will almost definitely need to be trapped - you won’t be able to pick her up and put her in a cage. Don’t even try it. You’ll need a humane trap, which many shelters will have and will let you borrow, or you can buy one for about $50.
The situation you are in is what happens when people are too g-d lazy to spay & neuter their pets and then can’t be bothered to take them to a shelter, even a kill shelter, when they don’t want them anymore. (Obviously I don’t mean you; I mean whoever used to have that mother cat.) 