gistfile1.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty

Builder.load_string('''

<MyContainer>:
    MyWidget:

<MyWidget>:
    my_button: my_button

    Button:
        id: my_button

''')


class MyContainer(BoxLayout):
    pass
    
class MyWidget(Widget):
    my_button = ObjectProperty()

    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        print "MyWidget#__init__ has been executed"

    def on_my_button(self, *args):
        print "MyWidget#my_button has been set"


class MyApp(App):
    def build(self):
        # return MyWidget()
        return MyContainer()

MyApp().run()

The answer I got in #kivy:

qua-non: kivy tries to delay the building of the gui of the widgets till they are actually needed. So the children inside a widget won't be set untill the next frame after super is called. When manually initializing a widget, properties can be initialised before the call to init/super

=> Yes, the behavior is expected!