how can you retrieve a forgotten restrictions passcode

That's a serious problem.

Use parental controls on your iPhone, iPad, and iPod touch - Apple Support

Unfortunately, as you can read in the above article, ... If you forget your Restrictions passcode....

Regards

Giulio

That's a serious problem.

If you forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode. Restoring your device using a backup won't remove the Restrictions passcode.

(extracted from Use parental controls on your iPhone, iPad, and iPod touch - Apple Support)

Regards

Giulio