CPT and metabox: create multi checkbox by using array?

There is no “fields” argument to add_meta_box(), which I assume is what you are using and not some code-bloat-ie helper function/class nonsense. And you’ve not identified a callback, which is the key. Create a callback and put your “warning” array in that, along with code to create your checkboxe/radio-boxes, or whatever you need.

$wpar_meta_box = array(
    'id' => 'series-post-meta-box',
    'title' => 'Information',
    'callback' => 'st_meta_box',
    'page' => 'stfic',
    'context' => 'normal',
    'priority' => 'high',
);
function st_meta_box($post) {
  $warn = array(
    'kid', 'violent', 'mature', 'adult'
  );
  // Code to create form markup
}