Guitar has certain advantages (like being portable and easily tuned), but I think it's easier to learn the basics of music on the piano. Everything is laid out in "single file", black and white. Piano is an easier instrument to handle physically too. At least you don't have to hold it while you play it! Guitars are bulky and awkward at first, and each hand must be trained quite differently.
Better than violin though -- at least a guitar has frets to guide intonation!   

A lot of my students start out on keyboards now, which has it's advantages. Keyboards never need tuning and take up much less space than a piano. I encourage parents to avoid the little "toy" models though, and opt for a mid-range keyboard with at least 5 octaves of regular-sized, touch-sensitive (ie you can play loud or soft) keys, and a minumum of distracting sounds and buttons. That way it will last the kids at least a couple years - but if they want to continue studying piano seriously of course they will eventually need the "whole" instrument!

Real pianos have a better action and a much more pleasing sound though - as long as they're properly maintained and tuned. You wouldn't believe how many families I've dealt with over the years who'd spend thousands on hockey equipment, video games and vacations without batting an eye, but try to convince them to upgrade or even spend $60 to tune and fix the old clunker Grandma sent all the way from Timbucktoo??? NOT!! *sigh* So many times I've heard "Oh I know it's not in good shape, but it's just fine to start on ..." Well, would you give your kid a rusty old bike with the wheels falling off it, tell him "ok kid, learn to ride!" and then expect him to like it???

Can't count how many times I've heard "Your piano sounds so much different than mine!" or "WoW - it sounds so much better when I play it here!" I tell them "yeah. It's because mine's in tune." It's frustrating to try to play an instrument when half the keys don't work, or when they do you hear 6 tones instead of one, or it sounds like doo-doo no matter what you do .... and they quit. So much for "it's fine to start on - and we'll get a nice piano in a few years if he's serious".    :-(

Anyway, thanks for letting me get that off my chest. 27 years of watching kids struggle with and give up on the old clunkers'll do it to ya. And one more thought - I think the easiest instrument for a child to learn is the recorder!

daylia