This blog post by a fellow piano teacher is wonderful, I can say that her advice here is invaluable. (Beta-tested it this summer.) 😉
This is wonderful reading for not only parents, but for older students, too. And piano teachers!
Here is the link. You can also download it here as a PDF to print it out to read it again and again.
How to Practice Anything
by Regina Ngo
Regina’s Music Studio, S. Pasadena, CA