Once you've [enabled Guided Access]( within your iPhone or iPad's accessibility options, you may have noticed that you can just as easily disable it as you can enable it from inside apps. If you don't want the person using your iPhone or iPad to be able to disable Guided Access on their own, you can set a passcode which will be required in order to exit Guided Access mode.
Launch the Settings app from the Home screen of your iPhone or iPad.
Tap on General.
Scroll down a ways and tap on Accessibility towards the bottom of the page.
Under the Learning section, tap on Guided Access.
Now tap on the option for Set Passcode.
You'll be asked to pick a 4-digit passcode and then reconfirm it by entering it one more time.
That's it. If someone attempts to exit Guided Access, they'll now have to know the passcode.