We have child-students in our school and whenever we go over techniques that could result in injury (wrist locks, arm bars, kicks to the knees, etc.), we STRESS that the techniques are dangerous and should only be done at full strength if they feel threatened with serious injury (ie. not playground horsing around). We do this everytime to make sure that they understand and remember.
So far (knock on wood), we have only had a few instances of in-class accidents resulting in injury and no cases that I know where a student has injured someone outside of class. The latter may be the result of student discretion or may be the result of not having any students so threatened.