Looked it up and it says:
Knock knees are caused from weak abductor muscles, which are found on the outsides of the hips and thighs. To correct this problem, do abductor-strengthening exercises.
Reccomended exercises are: Side Lunges, Side steps up and a couple more that are fully explained here http://www.livestrong.com/article/127712-exercises-correct-knock-knees/
and here http://livewell.jillianmichaels.com/therapists-exercises-knock-knee-4484.html