(requires that you have a rather outrageous and unusual chore at the ready in your mind.)
You are going to have fun with this one! Sometimes a person needs just to have an option in order to make the right choice. Here's how it works…You hear from your student "I don't want to practice today. " You respond very calmly and matter-of-fact, "OK, you don't have to practice. I have a different idea." You continue in that very calm and matter-of-fact way to explain to your student that often times in life we have choices.
"So here it is. You don't have to practice, you can do this chore instead." Child will generally ask what's the chore. That's when the fun begins for you! You go on to describe that all they need to do is the special chore, something such as sweeping the bathrooms, kitchen, patio/driveway, bagging everything up and taking it down to the trash.
Chances are they'll say, "do I HAVE to do that?" "Oh no, you can choose to practice instead, if you prefer."
This worked very well for us when our four sons were all learning piano at once, and we found it very entertaining for ourselves.