If you are using it with webix menu widget, you can construct the JSON structure for the menu with submenus included ( it will require a bit more complex php/sql logic )
If you want to show just a one level of menu and load sub-level only after the top menu item clicked, you can use a common menu or list widget, and data as you already have.
For such menu|list you can attach onItemClick handler, from which call server side data for the new dataset based on the clicked item ( id of clicked item and all data of item will be accessible in the onitemclick event handler )