you just bind a listener to the value change event
some_label = Label(...)
my_slider = Slider(...)
def OnSliderValueChange(instance,value):
some_label.text = str(value)
my_slider.bind(value=OnSliderValueChange)
as inclement points out in the .kv file you could do something like
<PongGame>:
...
canvas:
Rectangle:
...
Label:
...
text: str(slider_id.value)
Slider:
...
id: slider_id