from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
box = BoxLayout()
box.add_widget(Label(text='Hello world'))
box.add_widget(TextInput(text='Hi'))
popup = Popup(title='Test popup', content=box, size_hint=(None, None), size
    =(400, 400))
Kivy: Multiple items in a popup wont work