|Remember that we are working on dexterity for your hands more than we are working on strength. The strength necessary in the fingers will come as you practice your violin.
Here's dexterity exercise for violinists of all levels. You don't even need your violin to do the exercise!
To start, hold your hand with the palm facing up. Extend your thumb straight out from the hand. Now close the fingers, but not the thumb, into a fist. This is your starting position. Now straighten out the first and third fingers of the hand - without raising any of the other fingers. (I know, it's hard! But think how much you are helping your fingers to move independently!) You may not get them all the way out straight, but that's okay. Do your best. Hold this position for a count of 5. Now close fingers one and three fingers back into a fist. Relax for a moment. Now, straighten out your second and fourth fingers withouth moving the other two. When you have reached your maximum, hold the position for a count of 5, then close the fingers back into a fist. Repeat this cycle 10 times.
How does this help you? First, it helps you to move your fingers independently of one another. Second, it helps you to stretch the muscles so that each finger has a better range of motion. Third, it helps your concentration. Playing fingered double stops is not a natural motion. It requires practice. If a student can master this exercise while he is young, advanced playing will come much easier.