What can I do about abandoned kittens?

Some kittens got trapped in the crawlspace under our house. I heard them crying last night and the night before, but thought it was coming from outside. We opened the door to the crawlspace this afternoon and left it open so their mother could get them. I came home this evening and they were still there in the same places, hadn't been moved. I wrapped them in a hand towel and brought them into the backyard. The mother, a feral cat that hangs around sees them, but couldn't seem to care less. I finally put them back in the towel, put the towel in a box on its side, and put canned food out to encourage the mother cat to come back. She's sitting in the front yard now as though she doesn't have a care in the world. She hasn't touched the babies, but she also hasn't touched the food, which is unusual for her.

I know they're hers. I knew she had recently had kittens, but I didn't know she had moved them under the house while our plumbing was being worked on. They are very small, their eyes are open and their fur is fluffy, but they can't walk yet, only crawl.

We can't take them in. We are away from the house several hours a day, and we just adopted a 12 week old kitten last week.

I would take them to the humane society, but they won't be open until Monday.

Is she likely to take them back? Is there anything else I can do for them? I hate seeing them suffer.