And as noted above, many if not most music kids play at least some piano. I do think the piano has a really different focus, though; while a few kids do play in jazz or other ensembles, it is generally a much more solitary instrument. So I think it really depends on your/your child's goals for learning an instrument.
Yes, piano is a solitary instrument, so something else to consider. My kids have played the piano since they were 6 and really enjoy it. However, for them it is not a substitute for playing with other children in a band or orchestra. So you may find yourself paying for two instruments (piano and a second instrument). However, it is a great instrument to start with as others have said.
