kivy strange behaviour, child widget don't move with parent widget.

main.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.clock import Clock

class PongGame(Widget):

    def __init__(self, **kwargs):
        super(PongGame, self).__init__(**kwargs)
        Clock.schedule_interval(self.move, 1 / 60.)

    def move(self, *args):
        self.x += 1

class PongApp(App):
    def build(self):
        return PongGame()

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

pong.kv

#:kivy 1.0.9

<PongGame>:    
    canvas:
        Rotate:
            angle: 20
        Rectangle:
            pos: self.pos
            size: self.size

    sub_window: sub_window

    RelativeLayout:
        Widget:
            id: sub_window
            canvas:
                Color:
                    rgba: 1,0,0,1
                Rectangle:
                    pos: self.pos
                    size: self.size