You can use a mini library like objx - http://objx.googlecode.com/
You can write code like this:
var data = [ {"id":"10", "class": "child-of-9"},
{"id":"11", "class": "child-of-10"}];
// alert all IDs
objx(data).each(function(item) { alert(item.id) });
// get all IDs into a new array
var ids = objx(data).collect("id").obj();
// group by class
var grouped = objx(data).group(function(item){ return item.class; }).obj()
There are more 'plugins' available to let you handle data like this, see http://code.google.com/p/objx-plugins/wiki/PluginLibrary