BoxLayout inside of a RelativeLayout inside of a GridLayout positioning (updated with Button Functionality).

experiment.py

from kivy.uix.modalview import ModalView
from kivy.uix.gridlayout import GridLayout
from kivy.uix.relativelayout import RelativeLayout
from kivy.properties import StringProperty
from kivy.uix.behaviors import ButtonBehavior
from kivy.lang import Builder
from kivy.app import App
 
class ExampleGridLayout(GridLayout):
    pass
 
class ExampleButton(ButtonBehavior, RelativeLayout):
    text = StringProperty("When you click me, i don't respond.")
 
class MainView(RelativeLayout):
 
    def __init__(self, **kwargs):
        self.size_hint = (.5, .3)
        self.pos_hint = {'center_x': .5, 'center_y': .5}
        super(MainView, self).__init__(**kwargs)
 
        button_container = ExampleGridLayout()
        self.add_widget(button_container)
 
class TestApp(App):
    
    def build(self):
        root = MainView()
        return root
 
Builder.load_string("""
 
<ExampleButton>:
    state_image: 'atlas://data/images/defaulttheme/button'
    canvas:
        Color:
            rgb: 1, 1, 1
        Rectangle:
            size: self.size
        BorderImage:
            source: self.state_image
            size: self.size
        
    BoxLayout
        orientation: 'horizontal'
        #center_y: root.center_y
        pos_hint: {'center_x': .5, 'center_y': .5}
        size_hint: .75, .75
        Label:
            text: root.text
            shorten: True
 
<ExampleGridLayout>:
    cols: 1
    size_hint_y: None
 
    ExampleButton:
    ExampleButton:
""")
 
if __name__ == '__main__':
    TestApp().run()