How the app handles your credentials depends on how you exit from the app:

• If you simply close the app by whatever method works for your device (ex. double-clicking the home button and flicking the app upward on most Apple devices) then the app will remember your barcode and PIN.
• If you tap Logout from within the app then it will forget your barcode and PIN.

Why the difference? The app assumes that if you tap Logout then you either want higher security for your account or you want to log in to the app with another set of credentials (ex. your child's).