They are cute, thank you for sharing! I hope you'll be able to catch the mom so that she can get fixed. Some advice... if you can't afford to get the kittens their shots, vet treatment, and getting fixed, you give them to a rescue. I'm glad you are taking good care of them and have the opportunity to stop the cycle of stray cats at least with them.
One cat and it's partner can produce thousands of kittens in it's lifetime.
http://www.kinshipcircle.org/katrina/5_29_06.html