The muscles that low kicks rely on, among others, are the core muscles of the upper body, the hip flexors, and the muscle groups that make up the thigh (quadriceps, hamstring, etc.) as well as the muscles and tendons that stabilize the knee and ankle joints.
To get more accurate, try using a headache bag set low to the ground, at thigh height. This is basically a padded cylinder that has an elastic cord at top and another at bottom. These are attached to the floor and ceiling, or a tree limb, and the bottom attached to a weight of some kind that won't move. The height of the target is adjustable (up and down) so that you can work different attacks at different (correct) heights. The elasticity of the cords means that when you strike the target, it bounces away and snaps back, providing a dynamic, moving target for your attacks. These are relatively cheap - usually less than $30.00 - and can be found at many sporting goods stores which carry boxing or martial arts equipment. They are even cheaper and easier to find online. Work this target at mid-thigh height, and be sure to switch up your legs, as your opponent will not repeatedly expose the same side of their body as a target in actual practice.
For power, you must work something heavy and dense. Some suggestions are:
*A tire, mounted to something to keep it in place while you stomp, kick, and sweep.
*A heavy bag, with or without a retention cord on bottom to improve return speed and reduce swinging.
*A padded pole or post which is not apt to become structurally damaged.
*A Wing Chun Dummy, if you have the means, lol.
Where to aim: The mid thigh for roundhouse kicks, the back of the calf, where it is most muscled for sweeps, the quadriceps, knee, shin, or top of the foot for stomps.
*Watch Marco Ruas's fight against Paul Varelans (way back in UFC 7) for an example of the damage that repeated thigh kicks can do against a much bigger opponent.
*To properly execute a sweep (in the clinch) you must "pop" the muscle of your calf against the muscle of the opponent's calf. Almost a kick has to be used to knock the foot loose from the floor. A pulling, hooking type of motion will not work against anyone with experience or solid base/footwork.
*For roundhouse kicks, do not kick with the "ball of the foot", or with the shin. Instead, kick with the hard structure of the ankle complex. With your toes pointed so that the top of your foot is in nearly a straight line with your shin, kick with the bones which make up your ankle. Against a soft target suck as the muscle of the thigh, or the lower side, even ribs, the ankle will not become injured. Be careful kicking with these bones if you think there is a chance you will hit the opponent's knee, though, as this could damage your ankle. It is important to note that you need to kick with the "front" of your ankle, and not with the side, which would be painful for you as much as for the opponent.
Keep these basic principles in mind, train weekly for best results, and you will find that you are faster, strike harder, and more accurately within a short period of time. Good luck with your training, and let us know how it turns out!
22 years martial arts experience: Former blackbelt-level hand-to-hand combat / self-defense / personal protection / weapons instructor.