Kettlebells are awesome. In terms of efficiency, I believe the tool can not be beat. And just training with kettlebells alone will give you a strong midsection. Anyone who has performed hundreds of correct swings can vouch for how much the core must work during the movement, effectively transferring the kettlebell's energy back and forth. With that being said, most people should still include some extra "core" training.
When I say "core", most people immediately think about crunches and their many variations. Not me. The crunch analogous to the bench press in terms of real world strength carryover. Both are worthless. Besides, that repeated flexing of the spine is likely to create a disc herniation. Don't believe me? Start following the work of the most renowned spine biomechanist in the world, Dr. Stuart McGill. Spend some time on his site, or read his many books and published articles, then try to argue with me about how awesome you think your crunches are!
So if I am not talking about crunches, then how else can you train your core? By performing the many variations of planks and bridges silly! Throw a couple of sets of both in your kettlebell routine, or use them as part of your warm-up (you are warming up, aren't you?). Refer to my video below on how to perform your planks and bridges correctly.
The majority of your core training should center around exercises that lock the rib cage and pelvis together. Any separation between the two body parts will not only lead to injury, but will decrease your strength due to the energy leakage. Sure, you can still throw in your crunches from time to time, but keep them to a minimum. There are much better ways to stroke your ego!