If you want to retrieve the current displayed child of the ttk.Notebook, you do not need to use any extra data structures to map the tab index with the widget window.

Tkinter's nametowidget() method is available from all widget classes, so you can easily get the widget object that corresponds to a widget name:

    def select_tab(self, event):
        tab_id = self.notebook.select()
        frame = self.nametowidget(tab_id)
        # Do something with the frame