Remove then add widget to a BoxLayout

app2.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button
import time


kv = '''
BoxLayout:

    canvas:
        Color:
            rgb: 0.2, 0.2, 1
        Rectangle:
            pos: self.pos
            size: self.size

    Button:
        text: '1. Button'

    Button:
        text: '2. Button'

    Button:
        text: '3. Button'

'''

class App2(App):
    def build(self):
        loader = Builder.load_string(kv)
        first_button = loader.children[2]
        loader.remove_widget(first_button)
        loader.add_widget(first_button, index=2)
        loader.do_layout()
        first_button.canvas.ask_update()

        return loader

    def greet(self):
        print "Moving App!!!"

if __name__ == '__main__':
    App2().run()