Either keyboard or guitar is the logical first choice. Violin demands a much better sense of pitch - you have to "find" the correct pitches yourself, without help from either keys or frets, and it takes the average non-musician years to get any real music. (It took my wife weeks, but she started with an excellent background in piano and voice.) Either piano or guitar are easy to get to where you can at least accompany singing very quickly.

I am prejudiced in favor of the guitar, since I was given expensive piano lessons that didn't take, then was given a $15 guitar and learned on my own. I had told my parents that I wanted to do guitar in the first place, but they thought I should learn a "real" instrument. I would like to be able to play whorehouse piano, but not enough to put the work in.

I think the following arguments can be made for starting on guitar:

1) If you start the kid on nylon strings, it isn't very loud, and the suffering for the other people in the same house is minimized.

2) You can have your own guitar and take it places with you, and it feels more like "yours" than taking your chances with whatever piano is available. (This doesn't apply to electric keyboards, of course, but I assume we're talking about acoustic music here.)

3) Most courses of formal piano lessons concentrate on playing from sheet music to the extent that the kid never gets any concept of playing by ear until he stumbles onto it himself, if he does. (I'm over 50, so I get to follow the classic rule that the masculine 3rd person singular pronoun includes the feminine by implication. If you don't like it, show some respect for your elders and ignore it!) You can make music sooner if you start with chords and work toward melody, than the other way around.

4) The keyboard, with the black and white keys, teaches that the key of C major/A minor is "right" and all the other keys are abberations. (Did I spell that right? Probably not but to hell with it.) It's like there were "good" and "bad" notes within the 12 tone scale, when in fact the conventional major and minor scales are entirely arbitrary conventions that we of western european descent have gotten used to.

All of the above is simply my own opinion. Whether it's sufficiently humble is a matter of choice.

Peter.