People often mistake motivation for self-discipline. They believe motivation can keep you going for a long time toward their goals and are very disappointed when it doesn't. Motivation is the ignition, not the fuel in your engine. Your fuel is self-discipline, something that you do without questioning. The moment you start to question whether to do something difficult or not, there is a 90% likelihood you won't. This is precisely why a routine is important! I know that I absolutely hate the process of showering, doing my make-up and my hair every morning. Honestly, it is pure torture for me! But I do it every day without fail only because I have developed a habit and it is not a daily DECISION for me anymore. I just do it because I always do it.
Many kids start lessons (not just piano) because they enjoy music or think it would be so awesome to be able to play their favorite songs themselves. However, in order get to this goal of playing for fun, students need to practice regularly. And practicing, by definition, means that the student works on the pieces that are difficult, not the ones that they already know well. When a student sits down and plays the songs they already know, that is called playing for fun--which is the goal that we are trying to reach! :D
Now, maybe there is that 1 in 1000 kid that loves to practice the challenging parts (I know I wasn't one of them!) but the rest absolutely hate it! That is normal, and unfortunately we can't cure normal. Kids don't have the long-term view of life to understand that sacrifices now lead to great things later for them to enjoy (playing songs they want to play). Therefore, kids have a very hard time with self-discipline unless they get guidance from parents who help them set up routines.
Parents must help their kids in the same way whether it be for piano lessons, homework or sports. Studies show that it takes about 2 weeks to develop a habit of doing something without questioning it. Therefore, I suggest that parents take 2 weeks and enforce a practice schedule, even for just 5-10 minutes every day. Use of primary reinforcers (rewarding the student with a treat for complying with the schedule) will help navigate through complaining, whining and tantruming if that is an issue. At the end of 2 weeks, most parents will be pleasantly surprised that the kids sit down to practice without being asked, or with minimal reminders.
Of course, some students might not have that extra time in their schedule to devote to practicing. In this case, I would highly suggest that they post-pone taking lessons (whether an instrument or sport) until they have the time (even 10 minutes) every day to practice. When kids don't progress at an activity, they start to become bored, discouraged and even resentful! Many people think that making the kids practice will make them resentful to the activity, but on the contrary, they become more resentful when they don't seem to be getting anywhere. There are some students that make progress even just from weekly lesson and in that case, I would say it is ok to continue with lessons because they kids are happy with the goals they are reaching slowly but surely. In the end, what we want is for them to have a sense of achievement and the ability to play things that they enjoy.