Piano teachers hear this question a lot. I think it implies a more fundamental question: What is the nature of a child's wants and desires?
The answer to this question can lead to two different parenting approaches: One approach holds that every moment and decision of a child's day must be scheduled and controlled. The other approach says that a child should be allowed to do however he or she feels on the whim of the moment.
I disagree with both approaches, because I think they both deny the fundamental nature of what a child is. A child is a potential adult, but not a fully formed adult. A child is in the process of forming values, learning his or her likes and dislikes, and forming a full psychology.
So what does this mean?
A child's values and psychology aren't completely formed. This means that a child's decisions and emotions will often be at variance with his long-term self interest. For example, a child might want to eat sweets and candy all day. Or she might love the piano but not feel like practicing because it requires too much effort. In such cases, the parent is obligated to reduce the number of sweets, or encourage or even mandate practice.
On the other hand, a child's values and psychology aren't completely plastic. Although he isn't an adult, he does have some nascent values and psychology. To deny this fact, or thwart early attempts at self-discovery is to invite serious problems down the road.
For example, if a child has a clear preference for drawing or some other activity, and yet you deny this in favor of hours of daily piano practice, you're heading for disaster. In a case such as this, an "I don't want to practice" isn't an out of context whim, but rather the correct response to a value contradiction imposed from the outside.
The answer then to "What do I do if my child doesn't want to practice?" is: "It depends." If your child has other interests, and has had piano imposed from the beginning, you should consider letting her explore. If your child enjoys practicing in general but needs a day off, then let her chill out. If your child loves music but won't practice because of the effort involved, it could be time to find some creative motivation or be a strict parent. If your child doesn't want to do anything, it might be time to look for someone more qualified than a piano teacher for advice.