First and foremost choose the instrument that you or your child is most passionate about. What do you visualize yourself playing? Does your child play air guitar or air piano when music is present? If you are unsure, then I recommend starting with the piano. It is an easy basis for launching a fundamental knowledge of music. It is physically easier to play than the guitar. By this I mean that getting a clear tone from the piano is fairly simple as opposed to the guitar, which can cause some younger students frustration. The piano is linear, visually a little more comprehensible and immediately more satisfying. This has been my experience in teaching.