How to sign out of find my iPad

You can sign-in to iCloud/Find using a web-browser from any computer - and disable Find My iPhone from your account.

The procedure is described here:

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Remove your missing device from your account

If you have AppleCare+ with Theft and Loss, do not remove your lost iPhone from your account until your claim has been approved.

Go to appleid.apple.com to remove the missing device from your list of trusted devices.