• Is the instrument too challenging or is it not challenging enough (for both you and your child)?
• Does your child's temperament match the instrument?
• Can you afford the instrument and the maintenance that comes with it?
• As a parent, do you like the sound enough to listen to your child practice it for hours at home?
• Is your child specifically interested in a particular music style? If so, factor that into your instrument choice, as some specifically cater to certain styles. For instance, a violin player will have a hard time fitting in a jazz ensemble.
Experts don't always agree on which instruments are best for big kids to learn, but many music teachers do agree that it's hard to go wrong with the piano, percussion (like the drum or xylophone), recorder, guitar, or violin.