Forgot 4 digit restriction code
How to reset restriction code that I’ve forgotten on ipad
iPad, iOS 9
From: Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support
If you can't remember a restrictions passcode that you set on a previous version of iOS, or you forgot your Screen Time passcode, you need to erase your device, then set it up as a new device to remove the passcode. Restoring your device from a backup won't remove your passcode.
You'll need to restore the iPad, and you won't be able to sue a backup, since the restrictiosn passcode is stored in the backup.