To enable Safe Search go to Dashboard -> Web Filtering tab (top menu). Scroll down to the Additional settings section tick the Force Safe Search checkbox and click Save.
You can manage such settings as Force Safe Search, Force Restricted Mode for YouTube and others in the Additional settings tab.
With Force Restricted mode option enabled, YouTube will be forced to use Restricted Mode. This mode hides videos that may contain inappropriate content flagged by users. No filter is 100% accurate, but it should help you avoid the most inappropriate content. With the option enabled, YouTube will be accessible even if the Movies & Video category is blocked.
To force YouTube Restricted mode, go to Dashboard -> Web Filtering tab (top menu) and in the Additional settings section tick the Force Restricted Mode for Youtube checkbox and click Save.