Use $(this).children() instead of $(this).children() global query:
$(".thumb").hover(function() { $(this).children().show(); }, function() { $(this).children().hide(); });
Working demo: http://jsfiddle.net/h5x3f/2/
Note: if you are not concerned about Internet Explorer 6 support, you can completely eliminate jQuery / JavaScript and use the CSS :hover pseudo-class, which will even work with JS disabled. Or you can use a gasket like ie-7.js to handle :hover for you. See this version of your violin for an example.
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