Everything that has been said up to this point is so important. I understand your situation completely! I too fed ferals and strays on my porch and one was a female that kept bringing babies. That is where seven of my nine came from. I was never able to trap her and she has since disappeared. I saw many of her other babies get hit by cars or simply disappear. If you want these kittens you better hurry. It can be a hard task to teach them trust and with each passing day it gets harder.
The most important thing came from Jen. If at all possible trap the mother or any female for that matter and get them fixed! You can put them back out if you don't want to keep them or if they just refuse to be house kitties. The cycle will never end if you don't get them. I was ready to call in some professional trappers because it is heartbreaking to see what some of the kittens go through.
I wish you the best of luck. It is so rewarding to not only know that you have saved the babies but when you gain their trust and love it is awesome!
Keep us posted on your progress!! I get so excited about feral kittens!!!