If your feet cramp during exercises like Side Kicks, Footwork, or the Stomach Series, there may be a simple explanation for it. Its all in the way you point and flex your feet.
Test yourself by doing this: Sit on your mat with your legs straight ahead of you. Point and flex your feet at their maximum range of motion.
The key is to watch your toes- what are they doing? When you flex your feet, watch if your toes stretch back further than the top of your foot does. When you point, watch your toes and see if they curl tightly towards the arch of your foot. In both point and flexing, look at the tops of your feet, near your toes. You can probably see the long, thin bones leading to each toe straining up through the skin of your foot.
If you want to stop your feet from cramping, you'll have to re-train your feet on the proper way to point and flex.
Think about it: the point/flex motion is a folding and lengthening movement of your ankle. Its not a point/flex from the toes.
Fold at your ankle to draw the top of your foot back towards your leg. Watch your toes, they are probably still straining back. Try to relax your toes. In fact, try to flex less forcefully. It will feel like you're barely doing anything, but your toes will relax more easily. The same goes for pointing- point with less effort to keep ease in the foot.
Practice the new way of pointing and flexing as you sit with your legs forward so you can see what your toes are doing. Then, trying laying back and reach one leg up to practice. Imagine folding the ankle to push your heel away from your center. Lengthen the ankle to reach the top of your ankle away from your center. If you really focus, you can feel this new way of moving throughout your whole body, even your core.
Think of the way your foot looks as you stand on the ground. The ankle is flexed, but the toes are flat, at rest. They're not arching up and lifting off the floor.
This new way of moving your ankle and relaxing your toes will take a lot of practice, but the more you practice the more you will get used to it. And that's less cramping to deal with.
For more information on foot cramps, see my article, "Freedom From Muscle Cramps":