This is an easy thing to set up, only downside is that the student has to refresh the page after checking the completion box in the label before the quiz can become available.
It may be a versioning or browser thing - but this works as described in our Moodle - student checks label to complete, then the page reloads automatically and the restricted activity "appears".
One downside is that, unless the statement is then hidden somehow the student can uncheck it and the restricted activity disappears again!
One downside is that, unless the statement is then hidden somehow the student can uncheck it and the restricted activity disappears again!