How to add subitem in treewidget

[phillip_Qt]

Isn't it clear with function name itself "addChild" ?
Call addChild(), from the reference of the parent item. And new item will be added as child.

I think i didn't able to describe my problem properly. In my application, if i dbl click on tree widget a context menu ll pop up, having 2 option, either to add classroom or computers. after adding classroom i need to add some computers to a perticular classroom. I'm not able how to add the child. I've added my code. in my mainwindow i've amethod like addComputers()
here i m not able how to add the computers under the perticular class i've choosen.