So what you are talking about is a kick to the knee joint. This is done by striking the knee in a 45 or 90 degree angle usually from the inside or outside delivered to the side of the knee joint. This is a serious attack and should not be done in a school yard situation.

I'd suggest using the stop kick used to the mid/lower shin or lower thigh just above the knee, this attack/counter while the assailant is in motion can knock him to the ground or at least stop his forward progress, can cause temporay limping.

I'd suggest a quick lean away (guards up around your head) snapping kick that comes back quickly to avoid counters if you miss.