Even if their mom was still around, think of the life they would've had - not all would've survived, and the rest would've become just as wild - and would've continued the cycle of adding more poor stray kittens to the world. Maybe one thing that could still be done is see if a live trap could be set in that area (just during the day so you don't catch racoons or skunk). Mama needs help too!
If you contact your local humane society, they may know of a low cost spay/neuter clinic that could help too. I have an appt to fix our feral mom and young ones - hopefully the feral dad too if we can get him. The cost for all of them, including shots, will be about what it would cost to have ONE done at a local vet.
Thank you for caring!