My school used to have practiced sheets, where there was a set amount of time that was needed for every week. Your parents had to sign the paper after the week. Every week the teacher would collect everyones sheets and at the end of the month have a drawing and whoever was picked would get a prize. Most of the time the prizes were candy or a small funny toy. But you could make the reward anything to fit your kids. You could aslo have a contest with practicing, and whoever practed the most that week, they can run a small section of the class. It could be the part that the orchestra needs help with and if they have practiced it enough, they teach the class what they did to help them learn that section of music. It will let them feel like they are the teacher for a part of class.