I like having my legs as available as weapons as much as my upper limbs are. In another post I will show some defences against weapons that rely on quick feet or light legs.
This warm-up exercise is a great way to get the legs ready to deliver some powerful explosive kicks,
Start in a horse stance and step into a cross-stance. The leg with which you have stepped forward is now the supporting leg. Now from whichever side of your supporting leg the other leg's foot is resting, swing it over in a wide arc through an imaginary target at head height. The supporting leg is firm and the swinging leg is relaxed. The kicking foot should now come to rest at the other side of the supporting foot- to shoot of in another high, wide arc to the other side.
Repeat about 10 times each side and switch legs.
Real kicking would require you to tighten the kicking leg's muscles at the moment of impact. This exercise has none of that as it is meant to loosen the limbs before training.
Let me know how it works for you. :)