Thanks for the suggestions but re-starting didn't work - and let's be clear - even if it had that doesn't solve the issue: remote erase is supposed to work Remotely. There are lot's of people using iPads because of their overall very good security but if remote erase isn't working that's a serious bug.
It's still a bug and here's what;s causing it: attempting to erase an iPad from another iOS device that is on a different iCloud account but both are in the same Family Sharing group.
When you try this, not only does it not work, it prevents erasure from the iPad's own iCloud account!
Here is the work around until this bug gets fixed:
If you cancel the erase request from the iOS device you originally requested it from then you can re-request it from the iCloud account that the iPad you want erased is on. So always use iCloud on the web while logged into the same iCloud account.