In sparring, knees are rarely damaged by direct hits. This is because just about everyone trains to hit the thigh and not do any of the shots that would really wreck knees - you don't want to destroy your training partners. You'll note that most martial arts do not use knee protection for this reason.
The usual threats to knees are bad twisting motions while weighted -falling and throwing, though bad form with roundhouses and other movements could do it too. (Exception: playing around with heel hooks in grappling before people have control...) The typical protection worn for that is usually tight cloth/spandex knee braces - same as many other sports.
However, that alone isn't the answer. Better training of form, and solid stabilizer muscle training reduces the odds of taking those bad twists. Muscles stabilize joints, and the point of failure is always whatever muscle is either the weakest, or too poorly coordinated to activate in time. In those cases, it's not just "more weight/power", but rather a lot of work on unstable surfaces and shifting directional weight. Some traditional martial arts include some of this in their forms (especially all the stuff around balancing on one leg and slow kicks), but you'll get the most from working with wobble boards and any kind of modern stablizer work under a trainer.
This is months of work in the process, however, not a quick fix. If your group is incurring regular knee injuries, you are training unsafely and I would get in contact with better instructors and someone with sports medicine background.