There are many ways to go about selecting which instrument to learn about, and the links below are some examples that I have found on the net. Some of these methods are bizarre to me, but I have included them to provoke your comments.
1. The personality type method:
http://martiecoetser.hubpages.com/hub/A-music-instrument-suits-a-specific-personality-dont-choose-the-wrong-music-instrument-for-your-child
2. The physical method:
http://mrsaxde.hubpages.com/hub/Choosing-the-Right-Band-Instrument-for-Your-Child
3. The all-round method:
http://www.ehow.com/how_7841248_determine-musical-instrument-right.html
4. Probably the best answer so far:
http://voices.yahoo.com/what-musical-instrument-right-child-78038.html