Display QWidget over QVTKWidget

I have created a QWidget acting as parent(Lets say mainWindow). Its child is another QWidget named centralWidget. In centralWidget, i have a menu in leftSide. There are buttons on it, on clicking of a button, need to show QVTKWidget(child of centralWidget); and on click on another button, need to show QWidget(child of centralWidget).

On start of application, menu is shown in centralWidget. When i show QWidget, it is visible and then showing QVTKWidget, it is visible over QWidget. But when i do otherwise(show QVTKWidget first and then QWidget), then QWidget is not visible.(Here dont need hiding of QVTKWidget, since my QWidget is semi-transparent, need to see back Widget).

What to do to show QWidget over QVTKWidget?