I addressed this issue earlier, but I return, because it is an on-going concern. Both the teacher and the parents are key elements in the practice routine, at least with younger students.
I'll address the teacher first. How many times have your students made no progress between lessons? You know it's all about the practice. Talk to the child. Ask him or her what else is on the plate that keeps him from the piano. A lot of kids are over-scheduled any more. "If it's Tuesday, it must be piano" kind of week. Ask him why he takes lessons in the first place? Remind him that mom and dad are paying real money for the privilege (yes, it is a privilege) of taking lessons.
Does he like sports? Is he an athlete? Remind him how the team has to practice in order to improve; same with musicians.
Practice with her at the lesson, and write down exactly what she needs to do over the next week. Have her repeat the assignment to you, in mom or dad's presence. Do mom and dad understand the assignment? Write it out clearly, so there are no questions.
Ask him if there's a specific piece he wants to learn, and find it at his level. Not always as easy as it sounds; you might need to edit it a bit to fit his abilities.
Assign 3 or 4 short pieces she can read through at home, then have her pick her favorite piece to practice for the next lesson. What about that piece does she especially like?
Perhaps you need to lessen the assignment. Or have him keep a daily record of his practice time.
Parents, I have less suggestions for you. I actually don't like parents to push too hard; that often results in angry practice, which accomplishes very little. At least in my house, the more I pushed, the more she pushed back. My own daughter learned to hate the French horn.
Does she watch a lot of TV? Does she have an electric keyboard? She can play that during commercials. These days, I think there is more time spent airing commercials than the program.
Is the piano in a busy section of the house? He can run through a scale or a quick passage when he walks past the piano.
Even my committed adults have weeks where there really isn't time to practice. The lesson is a time to slow down, to enjoy the process of making music.
More than just a few thoughts. Sorry. My goal is not to make masters at the keyboard, but to encourage a love of music. Beat the rhythm on a drum, play the melody on a kazoo. And don't forget to laugh during your lessons. Remember, it's really not "work" you're asking him to do; he gets to "play" the piano. Such fun! Good luck.