Parents, Family, and Approved pickup contact types can always locate their check-in code from their account profile.
Codes will also by default appear on your mobile app home screen (shown below):
However, if you ever exit this check-in code banner from your home screen, you can always bring it back in your settings:
Log in on the app
Tap the horizontal lines ≡ menu (top left) > Edit profile
Scroll down and turn on 'Show check-in code'