iPad erase content

Bought son new iPad and looking to erase the content on the older iPad Mini 2. I was able to sign him out of the iPad via his apple id, however when I try to erase all content and settings I am running into issues. It is requiring a 4 digit passcode and we have tried everything. We are already up to 20+ passcodes we have tried and nothing is coming to mind on what in the world it might be. Any recommendations on how to get passed this so it erases all of the content so I can sell this iPad or trade it in? Thanks!