There is a question for an 8 year old, however for a younger child it might be different.
My child can not get through all the excercises and after 3 minutes walks away from the piano. Should I bribe her with ice-cream or money or other rewards?
What kind of practice are you trying? One of the key components of techniques like the Suzuki method is to have one parent sit down with the child during practice sessions. The point is that the child will want to do it if the parent shows some interest in the child's activity. If you just tell then to practice, turn on an egg timer, and then walk away (or worse, as everyone else has pointed out, bribe them), you turn what should be a joyous thing-- learning how to make music-- into an onerous chore that no one wants to do but everyone feels they should.
Our son really wants to play violin and piano, because he sees a lot of people around him doing it. My wife plays piano now at least three times a week, and he sits with her and mashes the keys. He's asked me to play (well, told me, because he's 3), and then proclaimed that my hands were broken because I couldn't. He's seen our neighbors' daughter play violin--she got accepted to college as a violin major-- and was just awestruck and really wants to try that too. He even sat still during Itzhak Perlman's performance on The Colbert Report, much to our amazement. Our neighbors have said that some of the most rewarding parenting experiences have been sitting with their daughter for an hour a day while she learns; personally, I'm looking forward to trying that.
Our daughter? Couldn't care less. Our neighbor's other kids? Also, could not care less about making music. Forcing them would probably result in their resentment, because they are just not interested.
I am the OP. I eventually used a bribe. One M&M lentil (or skittle) per song (page). In the first 4 months it worked. Later my child stopped requiring the reward and now can practice without the reward and loves the piano. So initial struggle was not worth it. You can bribe and fade it away later.
No bribes! If you reward your kids for an activity then the activity becomes an obstacle, something to be overcome to get what they want. By definition an obstacle is something to be overcome, avoided, etc.
My son, also 8, just started piano lessons. He asked for them which helps, but he has to practice every day for 10 minutes. We're not too strict on the time and sometimes it lasts as little as 3 minutes. We just stress that he do it every day. In the future we'll worry about making the sessions a little longer. Better he look at it as something interesting for a short time that something he dislikes for a longer time.
As someone you started Piano at 5 and took lessons on and off for 8 years, I would have to ask if the child wants to continue the piano.
Without knowing how mature your child is, or what she may have committed to before hand, I would suggest that piano is either 1) Not suited to her attention span, which should be managed, first 2) Not as much fun as she had thought and she would rather do other things 3) or Lastly, Important to her, but not she may not understand the importance of practice.
Looking back on my time in piano, I practiced far too little. On the one hand, I really only cared for music theory and the sound of the instrument, I never really enjoyed playing. If I had not been forced to continue, I probably would have taken it up, again, on my own, later. I would probably have also benefited if my staying in piano had been contingent on practice. Since both were 'mandatory', the less important one (to my parents: practice) suffered, and I never really benefited from the lessons.
There are also benefits in teaching your child to stick with something they don't like, but I don't think I learned that from being forced to stay in piano.
No bribes too.
I endorse what Brian says and add - make the practice one of the first things they do when they walk in door before anything else can distract them.
If any bribe - get an after school snack - apples or something cut up and ready for them as soon as they're finished. They're usually famished and would even eat broccoli at that point!
Just because "you" find it cool? Have you tried to find out whether your child is interested in any of the other above mentioned instruments?
Is she even interested in learning "any" kind of music, or her interest is Photography?
Well, the only kind of Piano I have seen is this. That is not something which one can actually hang on the back and go to college, or walk on roads, or play on bed or sofa, or play while moving.
Okay, maybe there are some smaller sized Pianos too, but my second point is that if she feels that she can actually "show off" her talent in a "stylish" and "unique" way to her friends and neighbors the very same day she learns a new lesson, may be that'll inspire her?