|Every student, every violinist is different. The approach to a lesson, practice session or a performance needs to take into account the personality, likes and dislikes of the student.
Effective rewards vary person by person. Learning varies person by person. What is good for Johnny may not be good for Mary and vice versa. Half the fun is figuring out what works (although until you figure out what works you may be in for a rather unpleasant period).
Do what works. Make it fun. That is the essence of great playing for young students.