Someone said it before, but it is worth repeating - a kick to the knee is most effective when the weight is on it. That's why they taught us to keep our feet moving in football - it's common to get hit in the knees. The simple defense against a knee kick is simply to raise that leg.
However, it seems that a lot of the "street fighting" these days uses almost a wrestler or boxer stance, which keeps the weight distributed on both knees, if not a little toward the front. If that's the case, distract them with a jab (but not too deep, you don't want him to lean back!) and take out the knee, either to the kneecap straight on, or a round kick to the side of the knee. If you get lucky enough to throw a side kick to the side of their knee, they won't be getting up on their own for weeks.