I think there is definitely a balancing act that you have to do as a parent in these types of situation. You can't make a kid love to play an instrument and as in eema's case, forcing kids to keep playing if they hate it can crush their love and enjoyment in music.
But for some kids, if you let them quit something when it becomes as challenge, they don't learn to meet challenges and grow from the experience. In my dd case, she still loved music but it started to get to the level of playing where she couldn't memorize the song the first/second time through, there were more elements she had to do all at once (more complicated bowings, new finger positions, more difficult rhythms and new types of notation in the music) and it was no longer 'easy' for her to play a piece well the first few times through. She had to break down the pieces and practice just a few measures at a time and I could see her frustration when she couldn't get it right away.
My husband thought we should just let her stop. But I felt really strongly that with a school situation that was completely not meeting her academic needs, she needed something that she would have to work at to see improvement. It took a lot of trial and error to find a system that worked for us, but by finding a reward system that encouraged (not forced) her to practice, she learned a valuable lesson and I don't just mean about playing the violin.
