Just triple click when you're done and enter the passcode to leave Guided Access code.

Any time you try to change the settings, or get more time on apps past the limit, you'll have to enter the passcode.

If you're setting the passcode on your own device, the prompt to enter the passcode might delay you enough to rethink your impulse.

You'll be prompted to enter the passcode if you try to turn off Screen Time, or go into an app past the time limit.