Piano is a very good instrument to start with. The theory is good for a strong musical foundation. I know a few kids who dropped out of piano around age 10 and later breezed through their AP Music theory course in high school - they told me that their time spent learning piano was well worth it for them.
With piano, your child can begin playing a piece that sounds "reasonably" OK within a short period of time. And your child can stick with it lifelong if he so prefers or use the musical background to move on to another instrument later. It is easier to pick up another instrument later because of the strong musical foundation laid by piano.
