In the case of my kids, it was easy for me to distinguish between hating to practice and hating to play the instrument. My D did have some crying spells early on, and still hates for me to critique her, but she loves to play.

My S hated practicing violin. When he told his violin teacher to her face that he hated everything about the violin, I had to take the hint and let him quit. He's happily playing a different instrument now.

I personally prefer instruments that allow kids to play in an ensemble setting (orchestra or band), particularly as they get into middle school and high school. My experience is that the interaction with other kids - and some friendly competition - can motivate them to practice and continue to play.

Neither of my kids started on piano. I didn't either - I started on a band instrument in middle school, and picked it up very quickly although I had not yet learned to read music. I played all the way through college.