Hi, I put a password on restrictions so my son couldnt change anything, but I have forgotten it. How can I fix this? thank you

Restore to a backup (if it was taken when the code wasn't on the iPad), or reset the iPad back to factory defaults (for both, first copy off anything that you want to keep) - to reset it back to factory defaults you may need to use recovery mode, there are instructions for doing that half-way down this page : If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support