If I don't know the unlock key for an Ipad how do I restore it to the factory settings?

There is a way of wiping the device I'm fairly sure it tells you on the apple website because if you permanently lock the device there must be a way of wiping it so try looking on the apple website or make a chat with a professional in the support area of the website or bring it into a apple store

If you don't have the device PIN and have access to none of the device and account recovery credentials that Apple users are prompted to set up when they start using a device or when they set up an account, then the answer is very simple:-

You don't have an iPad: you have a useless paperweight. It's actually a little bit better than that as it can still be a parts donor if the screen, battery, casing and if components like the home button(s), camera modules and similar are all in good condition.

You can try wiping it clean using another idevice.