One of the most common reasons parents give up on their children's lessons is due to fighting the child to practice. I promise, if you can find a way to get past this, it will be the secret to your child's music success.
How to battle. First you need to remember that you are in control and not the child. Dr. Suzuki's wife always said "your children will love you now and hate you later, or hate you now and love you later." Hate is a strong word (maybe there was something lost in translation...) I never "hated" my mother but I am SO glad that she stuck with me through the hard times and found creative ways to get me through. I can't tell you how many letters of appreciation I have written her over the years thanking her for fighting the fight and sticking with it.
Battle #1 Getting the child to the instrument
Solution #1 ~ make it routine- like brushing teeth, making their bed and eating breakfast. Do it at THE SAME time every day. If you have to miss a day of playing, do something in its place like theory or listening to some music--just make sure you call it "practice."