//получение моделей
$parent_id = (int)$_GET['id'];
$resource = mysql_query('select id, name from table  where parent_id = ' . $parent_id);
$result = array();
while(($row = nysql_fetch_assoc($resource))) {
    $result[] = $row;
}
echo json_encode($result);

$("#select_root").change(function(){

    var s = $(this);
    s.attr("disabled",true);
    $.ajax({
        url: "/parents.php",
        data: {id: s.val()},
        dataType: "json",
        success: function(data) {

             $_s = $("#some_select");
             $_s.html("");
             for (var i in data) {

                 var option = $("<option/>");
                  option.attr("value", data[i].id).html(data[i].name);
                  $_s.append(option);
             }
             s.attr("disabled", false);
        }
    })
});