We have this same fight with my 8 yr. old daughter. I don't require her to practice for a set number of minutes per day, because then it becomes "see how much time I can waste." I tell her how many times she must play each song. Usually twice, if it has been awhile 3 times. That way she controls how long she sits on the bench. If she sits down and does it quickly without fighting and whining, she's up in 10 minutes or so, if not she'll sit there for 30-40 minutes. She must play them correctly and with "heart," but not perfectly, becuase she is practicing to get better. I remember practicing as a kid and hated it. My reasoning on this was that she can control how long this takes and it is all on her and her attitude. Good Luck.