Cottontail rabbits become independent as soon as they leave the nest. If they are coming and going under the slab, they are self sufficient and can be caged or box-trapped and moved to another location. However, you will need to have the permission of the landowner or manager before releasing them. The distance of a mile or two should be enough to ensure that they will not find their way back to your place. However, your state may have specific regulations regarding trapping rabbits, requiring permits, specific traps, or other requirements. Check with the appropriate authorities (state wildlife agency, Cooperative Extension office, local animal control) before implementing any trapping program.
Cages or boxtraps are available for rent at some hardware stores and nature centers. Sliced apples work well as bait. Put a slice or two at the back of the cage, a couple in the middle, and a few morsels outside the trap in a trail to the entrance. Immature cottontails are fairly easy to catch. Do not set the trap where a captured animal can be exposed to direct sun, harassed by pets, or stolen by people. If you trap an animal, it is your responsibility to be aware of its well-being and care.
Closing the space under the slab, when the animals are removed or outside, will keep the next generation of bunnies or some other animals from extending your trapping chores. You might consider watching the animals leave the hole, closing it up (even temporarily), and letting them find a new place to hide. Momma bunny knows the area and probably has a secondary safe location already lined up. With them safely away, make your repairs permanent.
