Your json data has the wrong format, please use the 'loadFilter' function to convert your original data to correct data format.
$('#tt').treegrid({
loadFilter:function(data,parentId){
function getRows(children){
var rows = [];
function setRows(pid, children){
$.map(children, function(row){
row._parentId = pid;
rows.push(row);
if (row.children && row.children.length){
setRows(row.id, row.children);
}
row.children = undefined;
});
}
setRows(null, children);
return rows;
}
data.rows = getRows(data.rows);
return data;
}
})