Setting up an old ipad for mother-in-law

I am trying to clean up an old iPad (so the mother-in-law can use it). But when I attempt to 'reset' and 'erase all content and settings' it asks for my passcode (which I put in) but then it also asks for a 'restrictions passcode' which I don't know, and it isn't the same as my other passcode.

So I tried to reboot it whilst connected to my mac but that doesn't work. It just turns on again. I think extra security was set up when it was used once for work so that no one could make changes. Does anyone know how I can get round this without knowing this passcode?!

Thanks!