game.kv

<Game_Screen>:
    name: 'game_screen'
    on_enter: root.launch_game()

<Game_Menu>:
    FloatLayout:
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        size_hint: None, None
        canvas.before:
            Color:
                rgba: 1, 0, 0, 1
            Rectangle:
                pos: self.pos
                size: self.size
    Label:
        text: 'SPACE SHOOTER'
        font_size: 40
        #font_name: "fonts/Alexis Italic.ttf"
        color: 0, 1, 0, 1
        pos_hint: {'center_x': 0.5, 'center_y': 1.7}


    BoxLayout:
        canvas.before:
            Rectangle:
                pos: self.pos
                size: self.size
        size: self.parent.size
        pos: self.parent.pos
        orientation: 'vertical'

        Label:
            text: 'menu'
            #font_name: "fonts/Alexis Bold.ttf"
            font_size: 30
            color: 0, 1, 0, 1

        Button:
            text: 'start game'
            on_press: app.root.current = 'game_screen'
        Button:
            text: 'highest score'
            on_release:
        Button:
            text: 'credits'
            on_release: app.root.current = 'credits'

main.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.core.window import Window
from kivy.uix.button import Button

class Game(Widget):
     #  works fine on its own
    def __init__(self, **kwargs):
        super(Game, self).__init__(**kwargs)
        self.add_widget(Button(text="Gameplay, woo!"))



class Game_Menu(Screen):
    pass

class Game_Screen(Screen):

    def launch_game(self):
        game = Game()
        #Window.size = game.size
        #engines1.loop = True
        #engines1.play()
        self.add_widget(game)

class Menu_UI(Widget):
    pass

class Credits_Screen(Screen):
    pass

class GameApp(App):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(Game_Menu(name='menu'))
        sm.add_widget(Credits_Screen(name='credits'))
        sm.add_widget(Game_Screen(name='game_screen'))
        return sm

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