robottest.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.label import Label

class RootWidget(Widget):
    pass

class Robot(Widget):
	def on_pos(self, instance, value):
		print 'robot_pos: ', value

class CustomLabel(Label):
	def on_pos(self, instance, value):
		print 'label_pos: ', value

class TestApp(App):

    def build(self):
        pass

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

test.kv

RootWidget:

<RootWidget>:
	Robot:
<Robot>:
    pos: (250,250)
    size: (40,40)
    label_robot: robot_label
    canvas:
        Color:
            rgb: (1,0,0)
        Rectangle:
            size: self.size
            pos: self.pos
    on_pos: 
    	print self.pos
    CustomLabel:
        id: robot_label
        font_size: 25
        text: "B"
        color: (0, 1, 0, 1)
        pos: root.pos