def change_color(self, dt):
    for i in self.children:
        i.color.rgb = uniform(0.0, 1.0), uniform(0.0, 1.0), uniform(0.0, 1.0)


class GameApp(App):

    def build(self):
        return MyGrid()
if __name__ == '__main__':
    GameApp().run()
Position of widgets in GridLayout