The Dallas Symphony Association has a great website with information for teachers, children, and everyone! One of the articles on the kids page is all about practicing!
"Practice Makes Perfect??? - Not Always!"
"Every good musician knows that regular practice is a must, but did you know that careless practice can actually make you worse? Before you tell your parent or you teacher that you have decided not to practice, think about the WHO, WHAT, WHEN, WHERE and HOW of practice that will help you play better and enjoy music more."
You can read the rest of the article here.